Paint tab groups with the group color.

* The background of TabGroupHeader now uses the group color.
* The backgrounds of tabs in the group are tinted with the group color.

This treatment, along with the colors chosen, are intended to be
a placeholder.

Bug: 905491
Change-Id: Ic808548f8eba23064606e7fb8c9bba281d0d117f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610504
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Reviewed-by: Taylor Bergquist <tbergquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660498}
TabGroupData::TabGroupData() {
  static int next_placeholder_title_number = 1;
  title_ = base::ASCIIToUTF16(
      "Group " + base::NumberToString(next_placeholder_title_number));
   ++next_placeholder_title_number;
 
   static SkRandom rand;
  color_ = rand.nextU() | 0xff000000;
 }

TabGroupData::TabGroupData() {
  static int next_placeholder_title_number = 1;
  title_ = base::ASCIIToUTF16(
      "Group " + base::NumberToString(next_placeholder_title_number));
   ++next_placeholder_title_number;
 
   static SkRandom rand;
  stroke_color_ = rand.nextU() | 0xff000000;
 }
