[Telemetry] Fix race in tab.Navigate's script_to_evaluate_on_commit.

[Telemetry] Fix race in tab.Navigate's script_to_evaluate_on_commit.

I believe this is what was causing the page cyclers to hang on the bots. We
must add the script to evaluate on load prior to performing the navigation. If
we add it after, it is racy.

The reason it wasn't like this before is that Page.addScriptToEvaluateOnLoad
didn't work on cross-renderer navigations. But that problem is being fixed by
https://codereview.chromium.org/13949006/

The current file-based page cyclers don't do cross renderer navigations, so
will be fine even without the above fix. However, the above fix will be
necessary for the new WPR-based page sets.

BUG=None
TEST=page set based page cyclers on linux
NOTRY=True

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=194470

[dtu, 2013-04-16 21:25:57]

lgtm. Will this be a problem for the new WPR-based page sets on reference builds
in the future?

[tonyg, 2013-04-16 21:32:04]

On 2013/04/16 21:25:57, Dave Tu wrote:
> lgtm. Will this be a problem for the new WPR-based page sets on reference
builds
> in the future?

Yes. I need to update the reference builds after Pavel's fix :(

[nduca, 2013-04-16 21:34:27]

dumb drive by quesiton: what happens when the commit script raises an exception?
Does that propagate back into telemetry somehow?

[tonyg, 2013-04-16 21:44:05]

On 2013/04/16 21:34:27, nduca wrote:
> dumb drive by quesiton: what happens when the commit script raises an
exception?
> Does that propagate back into telemetry somehow?

No. It just silently fails at the moment. I'll file a bug and cc you.

[tonyg, 2013-04-16 22:33:34]

Committed patchset #1 manually as r194470 (presubmit successful).