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This adds a "pulse" animation to the send discovery dialog, which
gives a visual indication that scanning is underway to attempt to
discover other devices to share with.

This animation will also be added to the high-visibility receive dialog
in a separate CL.

Screenshot:
  https://screenshot.googleplex.com/4AYJTgtGcVHAuRi.png

Spec:
  https://carbon.googleplex.com/cros-ux/pages/nearby-share/motion

Bug: b:170890753
Change-Id: I7361021ba3ae4fbc03a01d055eb6ba10e5c65716
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2500279
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Reviewed-by: James Vecore <vecore@google.com>
Cr-Commit-Position: refs/heads/master@{#821433}