Revert of Add a mutex to GrContext::readSurfacePixels to protect against multiple CPU raster threads accessin… (patchset #1 id:1 of https://codereview.chromium.org/1329313002/ )

Reason for revert:
breaking the bots

Original issue's description:
> Add a mutex to GrContext::readSurfacePixels to protect against multiple CPU raster threads accessing the same GrContext to read back GPU input data
>
> BUG=chromium:524717
>
> TBR=reed@google.com
>
> Committed: https://skia.googlesource.com/skia/+/eb662bc407cec0585a821946fef123102cae64db

TBR=reed@google.com,reed@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:524717

Review URL: https://codereview.chromium.org/1334603002

include/gpu/GrContext.h

@@ -15,7 +15,6 @@
 #include "GrRenderTarget.h"
 #include "GrTextureProvider.h"
 #include "SkMatrix.h"
-#include "SkMutex.h"
 #include "SkPathEffect.h"
 #include "SkTypes.h"
 
@@ -381,11 +380,6 @@ private:
     bool                            fDidTestPMConversions;
     int                             fPMToUPMConversion;
     int                             fUPMToPMConversion;
-    // The sw backend may call GrContext::readSurfacePixels on multiple threads
-    // We may transfer the responsibilty for using a mutex to the sw backend
-    // when there are fewer code paths that lead to a readSurfacePixels call
-    // from the sw backend.
-    SkMutex                         fReadPixelsMutex;
 
     struct CleanUpData {
         PFCleanUpFunc fFunc;

src/gpu/GrContext.cpp

@@ -460,7 +460,7 @@ bool GrContext::readSurfacePixels(GrSurface* src,
     RETURN_FALSE_IF_ABANDONED
     ASSERT_OWNED_RESOURCE(src);
     SkASSERT(src);
-    SkAutoMutexAcquire ama(fReadPixelsMutex);
+
     // Adjust the params so that if we wind up using an intermediate surface we've already done
     // all the trimming and the temporary can be the min size required.
     if (!GrSurfacePriv::AdjustReadPixelParams(src->width(), src->height(),