Fix use of SkMutex.

Recent change to Skia moved SkMutex into its own header file. Include
it to use and SkMutex.

Use SK_DECLARE_STATIC_MUTEX to remove static initializer/finalizer.

Fixes master-skia build.

Change-Id: I2ab43c511587167b81310d5d55a65604d82761f5
1 file changed