Disable MediaHistoryBrowserTest.GetPlaybackSessions test

This test have been recently failing on Mac10.13 Tests (dbg)
Example build failure: https://ci.chromium.org/p/chromium/builders/ci/Mac10.13%20Tests%20%28dbg%29/17748
The flakiness seems to appear after crrev.com/c/2036772,
but it is difficult to cleanly revert.

TBR=beccahughes@chromium.org
Bug=1066853

Change-Id: I9444133768fd5c395922734336dc3cad371679ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2132187Reviewed-by: Anatoliy Potapchuk <apotapchuk@chromium.org>
Commit-Queue: Anatoliy Potapchuk <apotapchuk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755335}

chrome/browser/media/history/media_history_browsertest.cc

@@ -534,7 +534,8 @@ IN_PROC_BROWSER_TEST_P(MediaHistoryBrowserTest, DoNotRecordSessionIfNotActive) {
       GetPlaybackSessionsSync(GetOTRMediaHistoryService(browser), 1).empty());
 }
 
-IN_PROC_BROWSER_TEST_P(MediaHistoryBrowserTest, GetPlaybackSessions) {
+// Flaky failures: crbug.com/1066853
+IN_PROC_BROWSER_TEST_P(MediaHistoryBrowserTest, DISABLED_GetPlaybackSessions) {
   auto* browser = CreateBrowserFromParam();
   auto expected_default_metadata = GetExpectedDefaultMetadata();