v4l2: move v4l2_stub_header.fragment to media/gpu/v4l2/ folder

This CL moves the file v4l2_stub_header.fragment to
//media/gpu/v4l2/ folder.

This file is used in conjunction with the signature files
in the said folder, to produce an include file with the
functions used for v4l2 video acceleration. Right now having
the fragment in //media/gpu is unnecessary.

TEST= ** No new code added, if it compiles, we're good **
(Note that this code is compiled in the bot
chromeos_amd64-generic_chromium_compile_only_ng which is
part of the CQ).

This CL is kind-of a continuation of crrev.com/c/744462.

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
Change-Id: I11ea24e3feb27f53d06e3dcacccc2fdb0f0ac6fa
Reviewed-on: https://chromium-review.googlesource.com/747532Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513043}

media/gpu/BUILD.gn

@@ -69,7 +69,7 @@ if (use_vaapi) {
 
 if (is_chromeos && use_v4lplugin) {
   action("libv4l2_generate_stubs") {
-    extra_header = "v4l2_stub_header.fragment"
+    extra_header = "v4l2/v4l2_stub_header.fragment"
 
     script = "../../tools/generate_stubs/generate_stubs.py"
     sources = [