Re-enabling Firefox interop test.

The fix is now in the Firefox nightly; verified the test passes
locally.

Depends on https://webrtc-review.googlesource.com/c/webrtc.DEPS/+/3401.

R=ehmaldonado@chromium.org

Bug: 751211
Change-Id: I191db39747711b09a0c36e367e8e08fac0c53ea5
Reviewed-on: https://chromium-review.googlesource.com/681701Reviewed-by: Patrik Höglund <phoglund@chromium.org>
Reviewed-by: Henrik Kjellander <kjellander@chromium.org>
Commit-Queue: Patrik Höglund <phoglund@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504331}

chrome/browser/media/webrtc/webrtc_apprtc_browsertest.cc

@@ -291,9 +291,7 @@ IN_PROC_BROWSER_TEST_F(WebRtcApprtcBrowserTest, MANUAL_WorksOnApprtc) {
 }
 
 #if defined(OS_LINUX)
-// Disabled due to failure, see http://crbug.com/751211.
-#define MAYBE_MANUAL_FirefoxApprtcInteropTest \
-  DISABLED_MANUAL_FirefoxApprtcInteropTest
+#define MAYBE_MANUAL_FirefoxApprtcInteropTest MANUAL_FirefoxApprtcInteropTest
 #else
 // Not implemented yet on Windows and Mac.
 #define MAYBE_MANUAL_FirefoxApprtcInteropTest \
@@ -302,6 +300,7 @@ IN_PROC_BROWSER_TEST_F(WebRtcApprtcBrowserTest, MANUAL_WorksOnApprtc) {
 
 IN_PROC_BROWSER_TEST_F(WebRtcApprtcBrowserTest,
                        MAYBE_MANUAL_FirefoxApprtcInteropTest) {
+  base::ThreadRestrictions::ScopedAllowIO allow_io;
   DetectErrorsInJavaScript();
   ASSERT_TRUE(LaunchApprtcInstanceOnLocalhost("9999"));
   ASSERT_TRUE(LaunchColliderOnLocalHost("http://localhost:9999", "8089"));