Add text autosizing override in the inspector.

Add text autosizing override in the inspector.

This patch has two components:
1) Changes in the inspector to add a new text autosizing
override under the device metrics settings. This override
has been wired up to call methods on InspectorPageAgent.
2) Blink's Settings have been changed to support overriding the
text autosizing enabled boolean. A ternary enum has been added to
track whether the text autosizing feature is not overridden, overridden
as true, or overridden as false. Due to Blink's style guide, the
m_textAutosizingOverride variable must be declared as unsigned.

BUG=302005