GitLab

The WebAuth dialog has implemented its own MD-style pulldown button with
a down arrow on it. This CL moves the button class into its own file and
reuses it as the source picker pulldown button in the Cast dialog.

Bug: 893307
Change-Id: I565ce0e991c9f41f3b2ae6747e017dbbfec84523
Reviewed-on: https://chromium-review.googlesource.com/c/1278127
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604293}