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This change moves the test support class JavascriptTestObserver from
chrome/test to content/public/test. This test doesn't have any dependencies on
anything in chrome/, and moving it will allow tests in content to use this
utility class.

This class has to move to content to allow moving some Pepper testing from
chrome/ to content/

BUG=371873

Review URL: https://codereview.chromium.org/286243003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271209 0039d316-1c4b-4281-b951-d872f2087c98