GitLab

- Add MD shadows by...
  - Adding a member variable to BubbleBorder: std::Optional<int> md_shadow_elevation.
  - Modify static function GetBorderAndShadowFlags to support
    custom shadow elevation.
  - Adding a static function to cache ShadowValues.
    - I created a static function (GetShadowValues) here because other static
      functions assume that the default shadows are used.
  - Make GetBorderAndShadowInsets use gfx::ShadowValues::GetMargins when an elevation
    is set.

Bug: 826912
Change-Id: I93837da730fd5344aa026f67f1bff5ce3c766a98
Reviewed-on: https://chromium-review.googlesource.com/993275
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549379}