New tough_pinch_cases page set.

Add a new page set for pages that are tricky to pinch-zoom.

This CL also makes the pinch page action in Telemetry more flexible, for example
to perform a pinch-zoom on arbitrary elements of the page.

BUG=309484

Review URL: https://codereview.chromium.org/99943004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252625 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252625