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This fixes the background tab text color when using the Blackbird GTK theme.
Blackbird styles titlebar labels with dark text unless the label has the "title"
class [1].  This means creating a label in the titlebar will be illegible on
Blackbird since the frame is dark too.  This is a bug that was probably never
seen since having a non-title label in the frame is rare.

This CL adds ".title" to our selector to get the correct light color on
Blackbird.  Other themes should be unaffected.

[1] https://github.com/shimmerproject/Blackbird/blob/master/gtk-3.20/gtk-widgets.css#L4742

BUG=881550
R=pkasting

Change-Id: I16ef16c27ab4500c152bb56a105276be4bbf708a
Reviewed-on: https://chromium-review.googlesource.com/1217146Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590404}