Also ImageTransportFactoryBrowserTest.TestLostContext on Linux

These are currently crashing in VizProcessTransportFactory::GetGLHelper, a code path that should not be used.

The ImageTransportFactory test needs to be re-written for how it is handled in Viz

As discussed in triage I'm assigning to kylechar@

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/59d82432704fb58507671b44b0e1428efb129afb

commit 59d82432704fb58507671b44b0e1428efb129afb
Author: kylechar <kylechar@chromium.org>
Date: Mon May 28 19:53:19 2018

viz: Fix ImageTransportFactory tests with OOP-D.

For ImageTransportFactoryBrowserTest.TestLostContext to make it
compatible with OOP-D. The test doesn't need to use GLHelper /
OwnedMailbox to check if observers are notified about context loss.

For ImageTransportFactoryTearDownBrowserTest.LoseOnTearDown just early
exit for OOP-D. The test makes sure that ContextLossObservers get
OnContextLost() called on ImageTransportFactory destruction. This is
related to GLHelper so it's not needed with OOP-D.

Also don't compile the tests on macOS. They're disabled there anyways.

Bug:  785268 , 844469
Change-Id: I1c0513e0535bb841b4e8b843ebb54a97dcfe4ad8
Reviewed-on: https://chromium-review.googlesource.com/1066274
Commit-Queue: kylechar <kylechar@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562302}
[modify] https://crrev.com/59d82432704fb58507671b44b0e1428efb129afb/content/browser/compositor/image_transport_factory_browsertest.cc
[modify] https://crrev.com/59d82432704fb58507671b44b0e1428efb129afb/content/test/BUILD.gn
[modify] https://crrev.com/59d82432704fb58507671b44b0e1428efb129afb/testing/buildbot/filters/viz.content_browsertests.filter