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and Use ExtensionTestBrowserContext for ImageLoader unittest.

ExtensionsTestBrowserContext class does 
set TestExtensionsBrowserClient from constructor and
call DestroyBrowserContextServices from destructor.

ExtensionTestBrowserContext will be used instead of TestingProfile.

R=yoz@chromium.org, rdevlin@chromium.org
BUG=354046
TEST=unit_tests --gtest_filter=ImageLoaderTest.DeleteExtensionWhileWaitingForCache

Review URL: https://codereview.chromium.org/355223002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284330 0039d316-1c4b-4281-b951-d872f2087c98