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The session restore tests were timing out on Android. This was because
they were waiting for the background tabs to be loaded, which they
would not be in normal startup.

The only reason the test worked at all is that when devtools accessed
the background tabs this forced them to be loaded.

This changes the test to only wait for, and to only measure, the
foreground tab.

NOTRY=true
BUG=343168,343900

Review URL: https://codereview.chromium.org/161473002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251364 0039d316-1c4b-4281-b951-d872f2087c98