Revert "Speculatively re-enable TestVideoQualityH264."

This reverts commit fea48b7b.

Reason for revert: while not visible, this broke webrtc tests the 20th of August (see https://ci.chromium.org/p/chromium/builders/webrtc/WebRTC%20Chromium%20Mac%20Tester/8622)

Original change's description:
> Speculatively re-enable TestVideoQualityH264.
>
> This was reported as flaky. I am unable to reproduce, which could mean
> that the issue is fixed, or it could mean that it only flakes on certain
> setups (OS, builds, hardware, etc). To aid investigation I am
> speculatively re-enabling it; worst case it gets disabled again.
>
> Bug: 754684
> Change-Id: I866679464fce4facde927652751046e062ca1c81
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761602
> Reviewed-by: Guido Urdaneta <guidou@chromium.org>
> Commit-Queue: Henrik Boström <hbos@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#688531}

TBR=hbos@chromium.org,guidou@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 754684, 1007937
Change-Id: I3c6f8954b2a007e3a1ff3ac8db154c896a4cbebb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1823842
Commit-Queue: Armando Miraglia <armax@chromium.org>
Reviewed-by: Armando Miraglia <armax@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699722}

chrome/browser/media/webrtc/webrtc_video_quality_browsertest.cc

@@ -335,8 +335,15 @@ IN_PROC_BROWSER_TEST_P(WebRtcVideoQualityBrowserTest,
 
 #if BUILDFLAG(RTC_USE_H264)
 
+// Flaky on mac: crbug.com/754684
+#if defined(OS_MACOSX)
+#define MAYBE_MANUAL_TestVideoQualityH264 DISABLED_MANUAL_TestVideoQualityH264
+#else
+#define MAYBE_MANUAL_TestVideoQualityH264 MANUAL_TestVideoQualityH264
+#endif
+
 IN_PROC_BROWSER_TEST_P(WebRtcVideoQualityBrowserTest,
-                       MANUAL_TestVideoQualityH264) {
+                       MAYBE_MANUAL_TestVideoQualityH264) {
   base::ScopedAllowBlockingForTesting allow_blocking;
   // Only run test if run-time feature corresponding to |rtc_use_h264| is on.
   if (!base::FeatureList::IsEnabled(