Add CanAccuratelyCopyFromBackingStore() to the backing store class.
Add CanAccuratelyCopyFromBackingStore() to the backing store class.

Use the new method to avoid using CopyFromBackingStore() for screen captures when CopyFromBackingStore() will not give a flawless copy.

BUG=35204
TEST=Manual testing with example screenshot extension.

Review URL: http://codereview.chromium.org/830001

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@41475 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser