Remove unused GLHelper from content::ImageTransportFactory

Removed GetGLHelper() from ImageTransportFactory now that nothing is
using it. Also, removed some gl_helper.h #includes from modules where
it's not being used.

Bug: 870036
Change-Id: Ie632ddfbf9e73fe00db6992499b9d54fe529f4af
Reviewed-on: https://chromium-review.googlesource.com/1166219
Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581409}
11 files changed