Reland "Use Skia's new simplified read/write pixels code paths"

This reverts commit 835ae72c.

Reason for revert: Fix for MSAN issue landed here: https://skia.googlesource.com/skia/+/3d945e840c1955b5756fdeb696599c2818ed95fc

Original change's description:
> Revert "Use Skia's new simplified read/write pixels code paths"
> 
> This reverts commit 6d0559c0.
> 
> Reason for revert: Breaks ImageBitmapTest.AvoidGPUReadback on MSAN, e.g. https://logs.chromium.org/v/?s=chromium%2Fbb%2Fchromium.memory%2FLinux_MSan_Tests%2F9783%2F%2B%2Frecipes%2Fsteps%2Fwebkit_unit_tests%2F0%2Flogs%2FImageBitmapTest.AvoidGPUReadback%2F0
> 
> Original change's description:
> > Use Skia's new simplified read/write pixels code paths
> > 
> > Bug: 826819
> > Change-Id: Ia6eaca75fe43f9d9ec94f8fe644a44e8e7436675
> > Reviewed-on: https://chromium-review.googlesource.com/1054520
> > Reviewed-by: Robert Phillips <robertphillips@google.com>
> > Commit-Queue: Brian Salomon <bsalomon@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#557943}
> 
> TBR=bsalomon@chromium.org,robertphillips@google.com
> 
> # Not skipping CQ checks because original CL landed > 1 day ago.
> 
> Bug: 826819
> Change-Id: Ia4dca1186fbb79569672fbf5cdabd9657c702762
> Reviewed-on: https://chromium-review.googlesource.com/1056847
> Reviewed-by: Timothy Loh <timloh@chromium.org>
> Commit-Queue: Timothy Loh <timloh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#558215}

TBR=bsalomon@chromium.org,timloh@chromium.org,robertphillips@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 826819
Change-Id: Idd8043535288d6f224531f90d381f85ca548c01d
Reviewed-on: https://chromium-review.googlesource.com/1059793Reviewed-by: Brian Salomon <bsalomon@chromium.org>
Commit-Queue: Brian Salomon <bsalomon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558727}

skia/config/SkUserConfig.h

@@ -220,11 +220,6 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_SUPPORT_LEGACY_SVG_ARC_TO
 #endif
 
-// Remove after M66 branch to use new read/writePixels implementations
-#ifndef SK_LEGACY_GPU_PIXEL_OPS
-#define SK_LEGACY_GPU_PIXEL_OPS
-#endif
-
 #ifndef SK_SUPPORT_LEGACY_YUV_COLORSPACE
 #define SK_SUPPORT_LEGACY_YUV_COLORSPACE
 #endif