[login_manager] Enable --enable-impl-side-painting flag when using highdpi.
[login_manager] Enable --enable-impl-side-painting flag when using highdpi.

BUG=chromium:178317
TEST=local build, tested on daisy and link

Change-Id: Ied1954ea565ec392cb958224effd0091db262514
Reviewed-on: https://gerrit.chromium.org/gerrit/45618
Reviewed-by: Antoine Labour <9b5adc77717fb53f6e082b29cd49aa0d7f0ce7f0@chromium.org>
Commit-Queue: David Reveman <1dda8fffc4cb6d511b3f0060aad9cd12fd7f75f8@chromium.org>
Tested-by: David Reveman <1dda8fffc4cb6d511b3f0060aad9cd12fd7f75f8@chromium.org>

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