media: declare use_vaapi arg

The newer CrOS toolchains declare a new use_vaapi argument. Unless this
argument is declared in Chromium as well, gn gen will display this error
message:

ERROR at the command-line "--args":3:13: Build argument has no effect.
use_vaapi = true

The ninja files will be successfully generated nonetheless, but this is
still disturbing, so fix this by declaring this argument.

BUG=742069
TEST=Made sure Chromium was compiling with use_vaapi declared or not.

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: Ib10ac7d0762a09d13ba92391ba55585b786ad3f6
Reviewed-on: https://chromium-review.googlesource.com/579028Reviewed-by: Pawel Osciak <posciak@chromium.org>
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488196}

media/gpu/args.gni

@@ -9,4 +9,7 @@ declare_args() {
   # Indicates if Video4Linux2 codec is used. This is used for all CrOS
   # platforms which have v4l2 hardware encoder / decoder.
   use_v4l2_codec = false
+
+  # Indicates if VA-API-based hardware acceleration is to be used.
+  use_vaapi = false
 }