Reland "Enable vaapi decoding on linux x86/64 by default."

This reverts commit 6757c99.

Reason for revert: It was wrong culprit. Still failing here:
https://ci.chromium.org/p/chromium/builders/ci/Linux%20TSan%20Tests/59863

Original change's description:
> Revert "Enable vaapi decoding on linux x86/64 by default."
>
> This reverts commit 7bc2776.
>
> Reason for revert: A culprit that breaks
> WebRtcCaptureFromElementBrowserTest.CaptureFromMediaElement
> test on Linux TSAN:
> https://ci.chromium.org/p/chromium/builders/ci/Linux%20TSan%20Tests/59853
>
> Original change's description:
> > Enable vaapi decoding on linux x86/64 by default.
> >
> > Bug: 1097029
> >
> > Change-Id: Ib3105a11e5ad76f7ef22930246014a702e39d0dd
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436704
> > Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
> > Commit-Queue: Ted Meyer <tmathmeyer@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#817228}
>
> TBR=dalecurtis@chromium.org,tmathmeyer@chromium.org,andrescj@chromium.org
>
> Change-Id: I73a5d50738122ef5c81a0c7a92ec9016ac1420eb
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 1097029
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2474233
> Reviewed-by: Alexey Baskakov <loyso@chromium.org>
> Commit-Queue: Alexey Baskakov <loyso@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#817384}

TBR=dalecurtis@chromium.org,loyso@chromium.org,tmathmeyer@chromium.org,andrescj@chromium.org

# Not skipping CQ checks because this is a reland.

Bug: 1097029
Change-Id: I99ac6f3950373f1c0786c26795186787bff8b8bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2473865
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817422}

media/gpu/args.gni

@@ -16,7 +16,9 @@ declare_args() {
 
   # Indicates if VA-API-based hardware acceleration is to be used. This
   # is typically the case on x86-based ChromeOS devices.
-  use_vaapi = false
+  # VA-API should also be compiled by default on x11-using linux devices
+  # using x86/x64.
+  use_vaapi = is_linux && (current_cpu == "x86" || current_cpu == "x64")
 
   # Indicates if ChromeOS protected media support exists. This is used
   # to enable the CDM daemon in Chrome OS as well as support for