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}
float GM2TabStyle::GetHoverOpacity() const {
  const float range_start = float{GetStandardWidth()};
  const float range_end = float{GetMinimumInactiveWidth()};
  const float value_in_range = float{tab_->width()};
  const float t = (value_in_range - range_start) / (range_end - range_start);
  return tab_->controller()->GetHoverOpacityForTab(t * t);
}

float GM2TabStyle::GetHoverOpacity() const {
  const float range_start = float{GetStandardWidth()};
  const float range_end = float{GetMinimumInactiveWidth()};
  const float value_in_range = float{tab_->width()};
  const float t = (value_in_range - range_start) / (range_end - range_start);
  return tab_->controller()->GetHoverOpacityForTab(t * t);
}
