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}
int TabStrip::UpdateIdealBoundsForPinnedTabs(int* first_non_pinned_index) {
  layout_helper_->UpdateIdealBoundsForPinnedTabs(&tabs_);
  if (first_non_pinned_index)
    *first_non_pinned_index = layout_helper_->first_non_pinned_tab_index();
  return layout_helper_->first_non_pinned_tab_x();
}

int TabStrip::UpdateIdealBoundsForPinnedTabs(int* first_non_pinned_index) {
  layout_helper_->UpdateIdealBoundsForPinnedTabs(&tabs_);
  if (first_non_pinned_index)
    *first_non_pinned_index = layout_helper_->first_non_pinned_tab_index();
  return layout_helper_->first_non_pinned_tab_x();
}
