Revert of Disable RTCVideoRenderFrameCallbackNV12 test (patchset #1 id:20001 of https://codereview.webrtc.org/2951273002/ )

Reason for revert:
Test has been fixed.

Original issue's description:
> Disable RTCVideoRenderFrameCallbackNV12 test
>
> It has been crashing on trybots
>
> BUG=webrtc:7863
> TBR=magjed@webrtc.org
> NOTRY=True
>
> Review-Url: https://codereview.webrtc.org/2951273002
> Cr-Commit-Position: refs/heads/master@{#18720}
> Committed: https://chromium.googlesource.com/external/webrtc/+/4d25a0554a4bde9d76de5f3728fa181ba82fb0e1

TBR=kjellander@webrtc.org,oprypin@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:7863

Review-Url: https://codereview.webrtc.org/2959673002
Cr-Commit-Position: refs/heads/master@{#18758}
diff --git a/webrtc/sdk/objc/Framework/UnitTests/RTCMTLVideoViewTests.mm b/webrtc/sdk/objc/Framework/UnitTests/RTCMTLVideoViewTests.mm
index 17eaddf..63c59ff 100644
--- a/webrtc/sdk/objc/Framework/UnitTests/RTCMTLVideoViewTests.mm
+++ b/webrtc/sdk/objc/Framework/UnitTests/RTCMTLVideoViewTests.mm
@@ -187,8 +187,7 @@
   [test tearDown];
 }
 
-// The test is disabled because of frequent crashes. See bugs.webrtc.org/7863
-TEST(RTCMTLVideoViewTests, DISABLED_RTCVideoRenderFrameCallbackNV12) {
+TEST(RTCMTLVideoViewTests, RTCVideoRenderFrameCallbackNV12) {
   RTCMTLVideoViewTests *test = [[RTCMTLVideoViewTests alloc] init];
   [test setup];