Fix BMC 18260 very long page stops painting
Fix BMC 18260 very long page stops painting

Render process crashes when trying to capture thumbnail of cameinonsaturdays.com.
The root cause is that the height of this page more than 40000 pixels and cause
memory issue. Actually we should use the visible height of the page to
get thumbnail instead of actual height.

Reviewed by Ningxin


Former-commit-id: 737933bbaf3d53b2eafcefba0db86611eba0c17a
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser