Move TestRenderFrameHost, TestRenderViewHost and TestBackingStore to content\test as that's where test-only files go.

BUG=304341

Review URL: https://codereview.chromium.org/105143003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238993 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238993