Fix JavaScript wait expression in mobile_memory.json.

The previous approach was relying on the screen height/orientation and was
failing on Nexus 10. This new approach waits for a certain amount of images to
be displayed which works better across devices/screens.

TBR=tonyg@chromium.org

BUG=328686

Review URL: https://codereview.chromium.org/99243004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240864 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240864