Add custom useragent

This is an initial commit to improve anonimity in Nayuos R69+. It adds
the most common user agent string (Chrome R68 on Windows 10) to the
chromium process at startup.
This is also the first addition of a patching message to modify the
ChromiumOS code. It does not modify the chromium browser code, which
will be necessary for successive anonimity feature additions.

This patch compiles, but it has not been tested if the user agent is
actually modified when run on a board

software/nayuos/instance.cfg

@@ -99,7 +99,7 @@ context =
 recipe = slapos.recipe.template:jinja2
 template = {{ software_dir }}/scripts/cros_full_build.in
 rendered = ${directory:run}/cros_full_build
-md5sum = 2a64b5cc0314ec2221a949928956f5c0
+md5sum = ff09ba2bda748af933c78de4df9149fc
 mode = 0700
 context =
   key bash_path                         bin:bash
@@ -111,6 +111,7 @@ context =
   key keep_cache                        parameters:configuration.keep_cache
   key ebuilds_dir                       nayuos-ebuilds:location
   raw scripts_dir                       {{ software_dir }}/scripts
+  raw patches_dir                       {{ software_dir }}/patches
   raw logo_dir                          {{ software_dir }}/logo
 # packages to be copied from our overlay into ChromiuOS' overlay
 # those packages will be marked as dependency of root filesystem

software/nayuos/patches/user-agent.patch

+diff --git a/login_manager/chrome_setup.cc b/login_manager/chrome_setup.cc
+index cbe93f471..bd72cbb09 100644
+--- a/login_manager/chrome_setup.cc
++++ b/login_manager/chrome_setup.cc
+@@ -341,6 +341,9 @@ void AddSystemFlags(ChromiumCommandBuilder* builder) {
+   // On developer systems, set a flag to let the browser know.
+   if (builder->is_developer_end_user())
+     builder->AddArg("--system-developer-mode");
++  //Modify user agent to improve anonimity by preventing fingerprinting
++  const char* useragent = "$USERAGENTSTRING"
++  builder->AddArg(base::StringPrintf("--user-agent=%s",useragent))
+ }
+ 
+ // Adds UI-related flags to the command line.

software/nayuos/scripts/cros_full_build.in

@@ -152,6 +152,11 @@ increase_revision_number ${CHROMIUM_OVERLAY}/chromeos-base/openssh-server-init/
 sed -i -n '/src_install/q;p' $(latest_ebuild ${CHROMIUM_OVERLAY}/chromeos-base/chromeos-sshd-init)
 increase_revision_number ${CHROMIUM_OVERLAY}/chromeos-base/chromeos-sshd-init/
 
+# modify user agent to improve anonimity by preventing fingerprinting
+# TODO: scrape https://techblog.willshouse.com/2012/01/03/most-common-user-agents/
+NEWUSERAGENT="Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.106 Safari/537.36"
+sed -i "s!\$USERAGENTSTRING!${NEWUSERAGENT}!" {{patches_dir}}/user-agent.patch
+patch {{ cros_location }}/{{ branch }}/src/platform2/login_manager/chrome_setup.cc {{patches_dir}}/user-agent.patch
 
 # bashrc modifications
 BASH_EBUILD_DIR="${PORTAGE_STABLE}/app-shells/bash"

software/nayuos/software.cfg

@@ -6,6 +6,7 @@ extends =
   ../../component/libexpat/buildout.cfg
   ../../component/libaio/buildout.cfg
   ../../component/boost-lib/buildout.cfg
+  ../../component/patch/buildout.cfg
 
 parts +=
   slapos-cookbook
@@ -13,6 +14,7 @@ parts +=
   git
   curl
   thin-provisioning-tools
+  patch
 
 versions = versions
 
@@ -26,7 +28,7 @@ recipe = slapos.recipe.template:jinja2
 template = ${:_profile_base_location_}/instance.cfg
 rendered = ${buildout:directory}/instance.cfg
 mode = 0644
-md5sum = 73f964d73367dbf280d904e408a7f9b8
+md5sum = 30389eea4851d3e353466e1a2712c54e
 context =
   key software_dir            :_profile_base_location_
   key instance_dir            buildout:directory