Previously, when run under swarming, tab_capture_end2end_tests was
specified in gn_isolate_map.pyl as a 'gpu_test_launcher' rather than
as a 'windowed_test_launcher'. This, plus the way the target needed to
be declared in the BUILD.gn files was causing complications for analyze.

We think that there's no need for this separate kind of test_launcher,
as long as there is a way to tell MB to not actually launch xvfb. This
CL adds that support (by making xvfb.py be aware of a --no-xvfb flag)
and allows us to simplify a few things as a result.

Perhaps more interestingly, this'll allow us to actually get rid of the
console_test_launcher/windowed_test_launcher distinction altogether in
the future, but that's for a follow-up CL.

R=kbr@chromium.org
BUG=714336
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
NOTRY=true

Review-Url: https://codereview.chromium.org/2840953002
Cr-Commit-Position: refs/heads/master@{#467501}