Refactor how font scale factor appears in the emulation panel

This patch refactors the confusing font scale factor setting. A
screenshot is available at http://pr.gg/asnew.png

Changes included in this patch:
1) Added updated tooltips for DPR, text autosizing, and the use android
   text metrics checkbox. This patch also moves the tooltips
   to be on both the input element and the associated label.
2) Removes the font scale factor textbox. Instead of manually entering
   this value, it will automatically be calculated if the new
   "Use Android font metrics" checkbox is checked.
3) Adds a new "Use Android font metrics" checkbox and auto-check it
   based on which emulation preset is chosen. When text autosizing
   is disabled, this checkbox will be disabled.
4) Minor refactoring of metrics.fontScaleFactor() which now computes
   the factor on-demand (it's cheap).

Some usecases:
If an iOS device is emulated, the text autosizing box will be checked
but the android font metrics box will not.

If an Android device is emulated, both checkboxes are checked.

If a custom device is being emulated, the user can choose whether to
use android's font metrics.

BUG=319092

Review URL: https://codereview.chromium.org/73203003

git-svn-id: svn://svn.chromium.org/blink/trunk@162250 bbb929c8-8fbe-4397-9dbb-9b2b20218538