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We were not waiting for the file manager extension to initialize its
Web Worker process before starting the test, which lead to occasional
crashes if the test completed before the init was done.

BUG=chromium:89733
TEST=browser_tests --gtest_filter=FileManagerDialogTest.* --gtest_repeat=10 --gtest_shuffle

Review URL: http://codereview.chromium.org/7528003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@95875 0039d316-1c4b-4281-b951-d872f2087c98