Do not override text autosizing unless device metrics are overridden

This patch fixes a regression on Android devices where text autosizing
would be disabled if the inspector was attached but the device metrics
were not overridden.

This patch fixes the regression and adds a test of enabling/disabling
autosizing using the device metrics which failed before this patch.

Regressing patch:
http://src.chromium.org/viewvc/blink?view=revision&revision=160027

BUG=313079

Review URL: https://codereview.chromium.org/53803007

git-svn-id: svn://svn.chromium.org/blink/trunk@161223 bbb929c8-8fbe-4397-9dbb-9b2b20218538