Add flag --enable-fixed-position-compositing
Add flag --enable-fixed-position-compositing

BUG=None
TEST=Manual

Change-Id: Ib6f26c2d9ec132a8f243f420a6f0401e328e10fd
Reviewed-on: https://gerrit.chromium.org/gerrit/28595
Reviewed-by: Jon Kliegman <7addd549d0b04b2dc663a0a7eb2c1810caf11d96@chromium.org>
Reviewed-by: Zelidrag Hornung <2423ba2b026624ad482ad8e6887b2d657f9c0c43@chromium.org>
Reviewed-by: Alex Sakhartchouk <01a80683b775048b5be818465b24f13b15f52717@chromium.org>
Tested-by: Alex Sakhartchouk <01a80683b775048b5be818465b24f13b15f52717@chromium.org>
Commit-Ready: Ian Vollick <af2a47ae3b3794f18ca7a1d53b15ef079b7bbe2b@chromium.org>

chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager