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Starting chrome to about:blank instead of the NTP improves telemetry_unittests
cycle time by ~14% (172s -> 148s) on my MBP.

This required a couple of test updates:
1. It made testIFrame fail deterministically which exposed the source of
flake. The context_id isn't the current index, it is just a monotonically
increasing ID. So it depends on the number of contexts used previously.
2. It made the GPU memory test measure a larger delta (presumably due to the simpler previous page).

BUG=394454,388256

Review URL: https://codereview.chromium.org/466233002

Cr-Commit-Position: refs/heads/master@{#289423}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289423 0039d316-1c4b-4281-b951-d872f2087c98