Rename TextAutosizingFontScaleFactor to AccessibilityFontScaleFactor

There are currently two font scale factors in Settings:
1) deviceScaleAdjustment - represents the additional fudge factor
  applied to fonts in Android. In the future this may handle other
  scale adjustments.
2) textAutosizingFontScaleFactor - this is plumbed in as the
  accessibility slider value (under settings->accessibility in Chrome).

This patch contains two refactorings:
1) Rename TextAutosizingFontScaleFactor to AccessibilityFontScaleFactor.
2) Update the inspector emulation to override the deviceScaleAdjustment
  instead of the accessibility scale factor. In an unfortunate naming
  accident, the inspector uses "device scale factor" to mean DPR so I
  have left the override as "overrideFontScaleFactor". Note that this does
  have a change in behavior as the device scale adjustment is only
  applied in some situations whereas the accessibility scale factor is
  always applied.

This patch requires a two-sided change due to the API name change.

BUG=320193

Review URL: https://codereview.chromium.org/74663002

git-svn-id: svn://svn.chromium.org/blink/trunk@162255 bbb929c8-8fbe-4397-9dbb-9b2b20218538