mac: Disable ConstrainedWindowAnimations when running CHROME_HEADLESS.

We suspect that the private CoreGraphics APIs used by these animations
may be leading to WindowServer crashes. This uses simple window alpha
animations instead of those when running with CHROME_HEADLESS.

Bug: 828031
Change-Id: Ica1470315d7be946773f496647d65afc8759437c
Reviewed-on: https://chromium-review.googlesource.com/c/1273560
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598353}
1 file changed