Enable MediaSessionBrowserTest.{MultiplePlayers,Simple}PlayPause tests

These tests were disabled due to flakiness introduced by some recent
migrations from legacy IPC to mojo that landed past week, which I think
should be fixed by crrev.com/c/2587045, which has been just merged.

Let's re-enable them now to double-check if the fix actually worked
as expected, keeping an eye in case they are still flaky after this.

Bug: 1157263, 1157241, 1157239, 1157319
Change-Id: I2b1daffebbb157eca5f8cce3f7692dbfabe75bf8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593367
Auto-Submit: Mario Sanchez Prada <mario@igalia.com>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837643}

content/browser/media/session/media_session_browsertest.cc

@@ -247,14 +247,7 @@ IN_PROC_BROWSER_TEST_F(MediaSessionBrowserTestWithoutInternalMediaSession,
   EXPECT_TRUE(IsPlaying(shell(), "long-video"));
 }
 
-// Flaky on Linux and Android. http://crbug.com/1157239,
-// http://crbug.com/1157319
-#if defined(OS_LINUX) || defined(OS_ANDROID)
-#define MAYBE_SimplePlayPause DISABLED_SimplePlayPause
-#else
-#define MAYBE_SimplePlayPause SimplePlayPause
-#endif
-IN_PROC_BROWSER_TEST_F(MediaSessionBrowserTest, MAYBE_SimplePlayPause) {
+IN_PROC_BROWSER_TEST_F(MediaSessionBrowserTest, SimplePlayPause) {
   EXPECT_TRUE(NavigateToURL(shell(),
                             GetTestUrl("media/session", "media-session.html")));
 
@@ -272,15 +265,7 @@ IN_PROC_BROWSER_TEST_F(MediaSessionBrowserTest, MAYBE_SimplePlayPause) {
   EXPECT_TRUE(IsPlaying(shell(), "long-video"));
 }
 
-// Flaky on Linux and Android. http://crbug.com/1157239,
-// http://crbug.com/1157319
-#if defined(OS_LINUX) || defined(OS_ANDROID)
-#define MAYBE_MultiplePlayersPlayPause DISABLED_MultiplePlayersPlayPause
-#else
-#define MAYBE_MultiplePlayersPlayPause MultiplePlayersPlayPause
-#endif
-IN_PROC_BROWSER_TEST_F(MediaSessionBrowserTest,
-                       MAYBE_MultiplePlayersPlayPause) {
+IN_PROC_BROWSER_TEST_F(MediaSessionBrowserTest, MultiplePlayersPlayPause) {
   EXPECT_TRUE(NavigateToURL(shell(),
                             GetTestUrl("media/session", "media-session.html")));
 
@@ -332,10 +317,7 @@ IN_PROC_BROWSER_TEST_F(MediaSessionBrowserTest, MAYBE_WebContents_Muted) {
 
 #if !defined(OS_ANDROID)
 // On Android, System Audio Focus would break this test.
-
-// Flaky: http://crbug.com/1157263
-IN_PROC_BROWSER_TEST_F(MediaSessionBrowserTest,
-                       DISABLED_MultipleTabsPlayPause) {
+IN_PROC_BROWSER_TEST_F(MediaSessionBrowserTest, MultipleTabsPlayPause) {
   Shell* other_shell = CreateBrowser();
 
   EXPECT_TRUE(NavigateToURL(shell(),