Remove unneeded Mac SDK forward declaration.

BUG=1044828

Change-Id: I9a57f34570ef9b6ab332fed18dd5737de4c00e1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2016044
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734475}

base/mac/sdk_forward_declarations.h

@@ -37,9 +37,6 @@
 - (void)animationDidStop:(CAAnimation*)animation finished:(BOOL)finished;
 @end
 
-@protocol CALayerDelegate
-@end
-
 #endif  // MAC_OS_X_VERSION_10_12
 
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