Re-enable WebRtcRtpBrowserTests no longer flaky.

Enable two tests:
- WebRtcRtpBrowserTest.AddAndRemoveTracksWithoutStream
- WebRtcRtpBrowserTest.AddAndRemoveTracksWithIndividualStreams

Also remove the AppendSwitch for "RTCRtpSender", a flag which has been
removed and enabled by default.

TBR=phoglund@chromium.org

Bug: 788760
Change-Id: I2942aed11dca4bf70e4540840ff796def64a3f1d
Reviewed-on: https://chromium-review.googlesource.com/808384Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521670}

chrome/browser/media/webrtc/webrtc_rtp_browsertest.cc

@@ -22,8 +22,6 @@ class WebRtcRtpBrowserTest : public WebRtcTestBase {
 
   void SetUpCommandLine(base::CommandLine* command_line) override {
     command_line->AppendSwitch(switches::kUseFakeDeviceForMediaStream);
-    command_line->AppendSwitchASCII(switches::kEnableBlinkFeatures,
-                                    "RTCRtpSender");
     // Required by |CollectGarbage|.
     command_line->AppendSwitchASCII(switches::kJavaScriptFlags, "--expose-gc");
   }
@@ -78,8 +76,7 @@ IN_PROC_BROWSER_TEST_F(WebRtcRtpBrowserTest, GetReceivers) {
   VerifyRtpReceivers(right_tab_, 6);
 }
 
-IN_PROC_BROWSER_TEST_F(WebRtcRtpBrowserTest,
-                       DISABLED_AddAndRemoveTracksWithoutStream) {
+IN_PROC_BROWSER_TEST_F(WebRtcRtpBrowserTest, AddAndRemoveTracksWithoutStream) {
   StartServerAndOpenTabs();
 
   SetupPeerconnectionWithoutLocalStream(left_tab_);
@@ -275,7 +272,7 @@ IN_PROC_BROWSER_TEST_F(WebRtcRtpBrowserTest,
 }
 
 IN_PROC_BROWSER_TEST_F(WebRtcRtpBrowserTest,
-                       DISABLED_AddAndRemoveTracksWithIndividualStreams) {
+                       AddAndRemoveTracksWithIndividualStreams) {
   StartServerAndOpenTabs();
 
   SetupPeerconnectionWithoutLocalStream(left_tab_);