Reset to initial page scale in additional scenarios.

This patch causes page scale to reset to initial scale in two more
cases:
1) When the page-defined initial scale goes from -1 to a specific
value, or changes value.  This fixes a race where if page scale was reset
before the viewport tag was processed, then we would never go to
initial scale.
2) When the initial scale was set to a value less than the final minimum
scale and the content width expands.  In this case, the content width
was previously preventing us from going to the desired scale so we
should try again now that it's expanded.

Also no-op cleanup of some surrounding tests.

NOTRY=true
BUG= internal b/10935045, b/10838508

Review URL: https://codereview.chromium.org/26251010

git-svn-id: svn://svn.chromium.org/blink/trunk@161255 bbb929c8-8fbe-4397-9dbb-9b2b20218538