Add tooltips for font scale factor and device pixel ratio

This patch adds some minor UI tooltips for font scale factor and
device pixel ratio in the inspector's emulation view.

BUG=

Review URL: https://codereview.chromium.org/67743002

git-svn-id: svn://svn.chromium.org/blink/trunk@161692 bbb929c8-8fbe-4397-9dbb-9b2b20218538