[FastTextAutosizer] Incorporate accessibility and device scale factors

This patch incorporates the accessibility font scale factor and the
device scale adjustment into the multiplier calculation. For performance,
these scale factors are only accessed in prepareRenderViewInfo. In
addition, a debug-only assert has been added to ensure
prepareRenderViewInfo is called before the multiplier is calculated.

With this patch we pass 1 additional test!

BUG=333124
R=skobes
NOTRY=true

Review URL: https://codereview.chromium.org/135733002

git-svn-id: svn://svn.chromium.org/blink/trunk@165014 bbb929c8-8fbe-4397-9dbb-9b2b20218538