Avoid absoluteImageURL() creating data URLs for canvas by default.

This innocuous-looking call was changed in
https://codereview.chromium.org/255563004 to produce huge data URLs for
canvas elements.  This caused a performance regression in Android
WebView which needs to report URLs of the last tapped element to the
embedding app.  Switch absoluteImageURL() back to its previous behavior
of ignoring canvas elements, and introduce a new method
absoluteImageURLIncludingCanvasDataURL() that's only used in
WebViewImpl::saveImageAt and copyImageAt.

NOTRY=true
BUG=395859

Review URL: https://codereview.chromium.org/457013002

git-svn-id: svn://svn.chromium.org/blink/trunk@179888 bbb929c8-8fbe-4397-9dbb-9b2b20218538