Merge branch 'chromium'.

This introduces Chromium 39 as the new stable version along with a bunch
of fixes.

Fixes #2799, particularily the PDF plugin, which now is open source and
thus no longer an issue.

Also fixes #3219 and merges #2906, so we no longer get a crash while
trying to bring up the print preview dialog.

Thanks to @edwtjo for the CUPS version bump.

 * chromium: Switch to use open-source PDF plugin.
 * cups: bump 1.5.4 -> 1.7.5
 * chromium: Allow env vars for passing plugin paths.
 * chromium: Update all channels to latest versions.
 * protobuf: Clean up and update to version 2.6.1.