Roll Linux reference build to official build 31.0.1650.34 (trunk r224845, branch r230433)

README

 You want to be very careful about updating the reference builds, because it
 means changing the baseline all new builds are compared to on the performance
 graphs and can easily let regressions slip in. Ensure that the new builds that
 you use don't have any performance regressions by looking at the perf dashboard.
 
 
 IMPORTANT: When you update the reference builds, please send email to
            chrome-perf-sheriffs@google.com announcing the change, so that
            build.chromium.org's performance expectations will be reset.
 
 
 To update the reference builds:
 
   1. Select a target revision number that is green on all platforms.
 
-  2. Run src/tools/update_reference_build.py from a tmp directory. For example:
+  2. Run src/tools/update_reference_build.py from a tmp directory. To update a
+     Chromium reference build use -r <revision> option, for Chrome official
+     builds use -b <build_number>. To update a specific platform use
+     -p <platform> option. For example:
 
      $ cd /tmp
+     # Update a Chromium revision, example -r 123456
      $ /path/to/tools/update_reference_build.py -r <revision>
 
+     # Update a Chrome build, example -p Linux -b 30.0.1599.33
+     $ /path/to/tools/update_reference_build.py -p <platform> -b <build_number>
+
+
   3. Move to reference_builds/reference_builds and prepare a changelist.
      For example:
 
      $ cd reference_builds/reference_builds
      $ gcl change
      $ gcl upload <change>
      $ gcl commit <change>
 
   4. After your change is committed, roll the reference_build dependencies for
      each platform in src/DEPS.