Replace internal uses of BlinkMacSystemFont on Mac with system-ui

This patch replaces internal uses of "BlinkMacSystemFont", Blink's
non-standard extension on Mac, with the standard "system-ui" generic
font family.

By eliminating internal uses, the aliasing is moved from FontCache
to CSS resolving time. This allows access to the document, which is
needed for per-document use counter.

BUG=554590, 654679

Review-Url: https://chromiumcodereview.appspot.com/2388623002
Cr-Commit-Position: refs/heads/master@{#426743}
8 files changed