Plumb experiment flags through page_load_metrics and add a new observer

testing/variations/fieldtrial_testing_config_linux.json

Index: testing/variations/fieldtrial_testing_config_linux.json
diff --git a/testing/variations/fieldtrial_testing_config_linux.json b/testing/variations/fieldtrial_testing_config_linux.json
index 690552619f6da948138916a38ac8e56fc3e0730f..c68676022ad670e96ceb49827597d147bcffaeb3 100644
--- a/testing/variations/fieldtrial_testing_config_linux.json
+++ b/testing/variations/fieldtrial_testing_config_linux.json
@@ -49,6 +49,9 @@
                 "DocumentWriteEvaluator"
             ],
             "group_name": "Enabled"
+        },
+        {
+            "group_name": "Default"

[Ilya Sherman, 2016/04/07 18:46:04]

It's generally not recommended to include the default group in the test config,
as that doubles the test load, which doesn't scale.  If there are some specific
tests that you want to run in both configurations, then please update just those
test files.

[Charlie Harrison, 2016/04/07 19:12:52]

What about disabling the experiment completely for a particular platform? I'd
prefer that over cherry-picking certain tests to run in both configurations.

[Ilya Sherman, 2016/04/07 19:31:32]

Okay, I was apparently mistaken.  The bots will actually ignore any groups other
than the first one; so even if you specify the Default group here, it will be
ignored.  (Maybe we should actually make it a hard error to specify multiple
groups, since it would basically never do what the author expects, apparently.)

However, there is FYI bot coverage that will run tests with the Default group
for all trials.

So, if you'd like to have multiple groups tested on non-FYI bots, I don't think
there's any option available other than cherry-picking specific tests of
interest.

[Charlie Harrison, 2016/04/07 19:46:32]

The FYI coverage is okay with me! Thanks for the clarification.

         }
     ],
     "EnableGoogleCachedCopyTextExperiment": [