Add some shadow to input_alert.svg in order to match to input_alert.png.

Bug: 739091
Change-Id: I4dcb0617180fba917be85151601722f5e1cb40e9
Reviewed-on: https://chromium-review.googlesource.com/566790Reviewed-by: Keishi Hattori <keishi@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485603}