[DirectWrite] Limit noSubpixelForSmallSizeFont to low-DPI

Limit the noSubpixelForSmallSizeFont functionality that disables
subpixel text for older fonts at sizes below 16px to low-DPI devices.
At or above a 150% DPI subpixel text is enabled for all fonts assuming
anti-aliasing is enabled.

As the font code isn't aware of the DPI or device pixel ratio it needs
to be supplied by Chrome though WebFontRendering.

Chrome side change is in https://codereview.chromium.org/561503002/

R=cpu@chromium.org, dglazkov@chromium.org
BUG=405445

Review URL: https://codereview.chromium.org/553163002

git-svn-id: svn://svn.chromium.org/blink/trunk@181688 bbb929c8-8fbe-4397-9dbb-9b2b20218538