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}
TabStrip::TabStrip(std::unique_ptr<TabStripController> controller)
    : controller_(std::move(controller)),
      layout_helper_(std::make_unique<TabStripLayoutHelper>()),
      drag_context_(std::make_unique<TabDragContextImpl>(this)) {
  Init();
  SetEventTargeter(std::make_unique<views::ViewTargeter>(this));
  md_observer_.Add(MD::GetInstance());
}

TabStrip::TabStrip(std::unique_ptr<TabStripController> controller)
    : controller_(std::move(controller)),
      layout_helper_(std::make_unique<TabStripLayoutHelper>()),
      drag_context_(std::make_unique<TabDragContextImpl>(this)) {
  Init();
  SetEventTargeter(std::make_unique<views::ViewTargeter>(this));
  md_observer_.Add(MD::GetInstance());
}
