Re-enable placeholder-shown-style-update.html on ASAN

The CL that was causing this test to crash on ASAN was reverted.
Re-enable the test.

Bug: 1121429
Change-Id: Id7d6a6f134424d3953a2aab2617e3a45ac1d6de1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3071947


Commit-Queue: Daniel Libby <dlibby@microsoft.com>
Reviewed-by: Daniel Libby <dlibby@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#908700}