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This adds sk_memory_barrier(), implemented using sk_atomic_fetch_add() on an uninitialized variable.  If that becomes a problem we can drop this to the porting layer, using std::atomic_thread_fence() / __atomic_thread_fence() / __sync_synchronize().

The big win is that ref() doesn't generate a memory barrier any more on ARM.
This is an instance of SkSafeRef() in SkPaint(const SkPaint&) after this CL:
     4d0:       684a            ldr     r2, [r1, #4]
     4d2:       6018            str     r0, [r3, #0]
     4d4:       b13a            cbz     r2, 4e6 <_ZN7SkPaintC1ERKS_+0x2e>
     4d6:       1d10            adds    r0, r2, #4
     4d8:       e850 4f00       ldrex   r4, [r0]
     4dc:       3401            adds    r4, #1
     4de:       e840 4500       strex   r5, r4, [r0]
     4e2:       2d00            cmp     r5, #0
     4e4:       d1f8            bne.n   4d8 <_ZN7SkPaintC1ERKS_+0x20>

Here's the before,  pretty much the same with two memory barriers surrounding the ref():
     4d8:       684a            ldr     r2, [r1, #4]
     4da:       6018            str     r0, [r3, #0]
     4dc:       b15a            cbz     r2, 4f6 <_ZN7SkPaintC1ERKS_+0x3e>
     4de:       1d10            adds    r0, r2, #4
     4e0:       f3bf 8f5f       dmb     sy
     4e4:       e850 4f00       ldrex   r4, [r0]
     4e8:       3401            adds    r4, #1
     4ea:       e840 4500       strex   r5, r4, [r0]
     4ee:       2d00            cmp     r5, #0
     4f0:       d1f8            bne.n   4e4 <_ZN7SkPaintC1ERKS_+0x2c>
     4f2:       f3bf 8f5f       dmb     sy

The miscellaneous files in here are just fixups to explicitly include SkMutex.h,
instead of leeching it off SkRefCnt.h.

No public API changes.
TBR=reed@google.com

Build trybots seem hosed.
NOTRY=true

BUG=skia:

Review URL: https://codereview.chromium.org/896803002