[GTK] Add title class to background tab text color selectors

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}

chrome/browser/ui/libgtkui/gtk_ui.cc

@@ -947,9 +947,9 @@ void GtkUi::LoadGtkValues() {
           SK_ColorTRANSPARENT;
 
       const SkColor background_tab_text_color =
-          GetFgColor(header_selector + " GtkLabel");
+          GetFgColor(header_selector + " GtkLabel.title");
       const SkColor background_tab_text_color_inactive =
-          GetFgColor(header_selector_inactive + " GtkLabel");
+          GetFgColor(header_selector_inactive + " GtkLabel.title");
 
       color_map[ThemeProperties::COLOR_BACKGROUND_TAB_TEXT] =
           background_tab_text_color;