Add API for GrContext to recommend rendertarget sample count

Add API for GrContext to recommend rendertarget sample count

Add GrContext::getRecommendedSampleCount method that can be
used to determine which GPU backend and what exact sample count
is recommendeded for a particular render target at particular
dpi.

Make this initially recommend 4xMSAA for contexts which have
NVPR enabled if dpi is 250 or more, 16 if dpi is less than 250
and no MSAA for others.

BUG=chromium:347962
R=bsalomon@google.com

Author: kkinnunen@nvidia.com

Review URL: https://codereview.chromium.org/188403003

git-svn-id: http://skia.googlecode.com/svn/trunk@13717 2bbb7eff-a529-9590-31e7-b0007b416f81