Enable "system-ui" generic font family

This patch enables the "system-ui" generic font family on all
platforms.

Following changes are planned but not included in this CL:
1. Content sets the current system font to Blink on Linux/CrOS[1].
2. Remove internal use of "BlinkMacSystemFont" and add UMA[2].

[1] https://codereview.chromium.org/2138613002
[2] https://codereview.chromium.org/2388623002

BUG=654679

Review-Url: https://codereview.chromium.org/2137483004
Cr-Commit-Position: refs/heads/master@{#425967}
17 files changed