Factor text size device mapping in SkScalerContext.

Factor text size device mapping in SkScalerContext.

All of our font back-ends use the text size in some way other than
simple concatentation with the current matrix. The code here finds
the full device matrix and then decomposes it to extract the text
size.

FreeType, GDI, and DirectWrite use the text size as the pre-hint scale.
CoreText will not scale color emoji above the requested text size,
and certain features like 'trak' are performed on the text size.

Review URL: https://codereview.chromium.org/748883005