Fix a memory leak reported by clang static analyzer.

Fix a memory leak reported by clang static analyzer.

The CFStringCreateWithCString method respect the "Create Rule" [1]
regarding the ownership of the returned reference. This means that
the object needs to be deallocated explicitly by calling CFRelease.

[1]: https://developer.apple.com/library/content/documentation/CoreFoundation/Conceptual/CFMemoryMgmt/Concepts/Ownership.html#//apple_ref/doc/uid/20001148-103029

BUG=648210
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2354963002

Review-Url: https://codereview.chromium.org/2354963002