Added UMA tracking of mobile vs. desktop pages.

Added UMA statistics to track what proportion of page loads are mobile
designed sites vs desktop. This is accomplished by tracking the viewport
specifications on a page (meta viewport, mobileoptimized meta,
handheldfriendly meta, and XHTML-MP doctype). We bucket the type of
viewport specified, and in the case of a fixed width viewport meta we 
track the zoom required to see the whole page on the device's screen.

See this document for details: https://docs.google.com/document/d/12-FpZTCOUAWVstrfL_Z9kujl9PaRPymlnm9KAO6Rw7Q/edit?usp=sharing

CL for histograms.xml: https://codereview.chromium.org/212553006/

BUG=329697

Review URL: https://codereview.chromium.org/216183004

git-svn-id: svn://svn.chromium.org/blink/trunk@173392 bbb929c8-8fbe-4397-9dbb-9b2b20218538