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This CL extends task throttling algorithm to multiple task case.
Comparing to previous algorithm, new one backs off the throttling duration by elapsed time.
So that consecutive failure doesn't throttle the next task too long time.


BUG=344769
TEST=unit_tests --gtest_filter="SyncProcessRunnerTest.*"

Review URL: https://codereview.chromium.org/386523002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282934 0039d316-1c4b-4281-b951-d872f2087c98