Avoid calling notifyResizeForViewportUnits when viewport wasn't resized.

This is a first CL in fixing the cause for a layout and onerror storm that are triggered by 'w' descriptor broken images.
This CL prevents notification on viewport changes when the viewport has in fact have not changed. 
Before this CL, these notifications were called upon every layout.

BUG=410268

Review URL: https://codereview.chromium.org/533893005

git-svn-id: svn://svn.chromium.org/blink/trunk@181344 bbb929c8-8fbe-4397-9dbb-9b2b20218538