system_media_capture_permissions_mac.mm: Fix compile failures in the latest Clang

Recent Clang versions started diagnosing incorrect conversion from 'Class' to
object pointer types.

Bug: 1017213
Change-Id: I3978fe4d0bc338b1ff5b7a3fdcf5029e209ac037
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879460Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709543}

chrome/browser/media/webrtc/system_media_capture_permissions_mac.mm

@@ -56,7 +56,7 @@ class MediaAuthorizationWrapperImpl : public MediaAuthorizationWrapper {
 
   NSInteger AuthorizationStatusForMediaType(NSString* media_type) final {
     if (@available(macOS 10.14, *)) {
-      AVCaptureDevice* target = [AVCaptureDevice class];
+      Class target = [AVCaptureDevice class];
       SEL selector = @selector(authorizationStatusForMediaType:);
       NSInteger auth_status = 0;
       if ([target respondsToSelector:selector]) {
@@ -77,7 +77,7 @@ class MediaAuthorizationWrapperImpl : public MediaAuthorizationWrapper {
                                  base::RepeatingClosure callback,
                                  const base::TaskTraits& traits) final {
     if (@available(macOS 10.14, *)) {
-      AVCaptureDevice* target = [AVCaptureDevice class];
+      Class target = [AVCaptureDevice class];
       SEL selector = @selector(requestAccessForMediaType:completionHandler:);
       if ([target respondsToSelector:selector]) {
         [target performSelector:selector