CL currently out for review: https://codereview.chromium.org/2783673003/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/377f6d9ede58daddce19df03bbb2a280c4afc550

commit 377f6d9ede58daddce19df03bbb2a280c4afc550
Author: pbos <pbos@chromium.org>
Date: Fri Apr 21 12:50:28 2017

Separate stubs for V4L2 and VAAPI.

Permits building Chromium with both V4L2 and VAAPI. Before this change
when building with use_v4lplugin set on non-arm platforms
generate_stubs.py would be called to generate two InitializeStubs()
functions (and other symbols) under the media_gpu namespace, breaking
the one-definition rule and failing to link both VAAPI and V4L2.

R=posciak@chromium.org
BUG= 709175 
TEST=Compile Zako with use_v4lplugin.
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

Review-Url: https://codereview.chromium.org/2783673003
Cr-Commit-Position: refs/heads/master@{#466319}

[modify] https://crrev.com/377f6d9ede58daddce19df03bbb2a280c4afc550/media/gpu/BUILD.gn
[modify] https://crrev.com/377f6d9ede58daddce19df03bbb2a280c4afc550/media/gpu/generic_v4l2_device.cc
[rename] https://crrev.com/377f6d9ede58daddce19df03bbb2a280c4afc550/media/gpu/v4l2/v4l2.sig
[rename] https://crrev.com/377f6d9ede58daddce19df03bbb2a280c4afc550/media/gpu/vaapi/va.sigs
[rename] https://crrev.com/377f6d9ede58daddce19df03bbb2a280c4afc550/media/gpu/vaapi/va_drm.sigs
[rename] https://crrev.com/377f6d9ede58daddce19df03bbb2a280c4afc550/media/gpu/vaapi/va_x11.sigs
[modify] https://crrev.com/377f6d9ede58daddce19df03bbb2a280c4afc550/media/gpu/vaapi_wrapper.cc