Set the font family for the "system-ui" generic font family on Linux

This patch sets the font family for the "system-ui" generic font family
added in [1] on Linux.

With this change, all platforms use the correct font. On Windows, the
browser already pass menu_font_family_name to the renderer. On Mac and
Android, the render can find the system font without needing the
browser to set.

On Linux and Chrome OS, gfx::Font() calls LinuxFontDelegate to figure
out the current system font. On the default Ubuntu, this resolves to
"sans", which Skia resolves to "DejaVu Sans", or "Arial" in
run-layout-test.

[1] http://crrev.com/2137483004

BUG=654679

Review-Url: https://codereview.chromium.org/2138613002
Cr-Commit-Position: refs/heads/master@{#427262}
10 files changed