Reland "Use new Skia GPU read/write pixel code paths"

This reverts commit 56044665.

Reason for revert: Fast paths for premul/unpremul RGBA added to new code paths to accelerate get/putImageData.

Original change's description:
> Revert "Use new Skia GPU read/write pixel code paths"
> 
> This reverts commit f17c70fe.
> 
> Reason for revert: 
> 
> It looks like this change:
> 
> slowed down the file___static_top_25_weather.html rasterize_and_record time on the AndroidOne (crbug.com/825594)
> 
> slowed down getImageData and putImageData for the HTML5 Canvas across the board (crbug.com/825585)
> 
> Bug: 825594
> Bug: 825585
> 
> Original change's description:
> > Use new Skia GPU read/write pixel code paths
> > 
> > Change-Id: I8ea66d1b3bb8db174deab690a0cdacc39cf81992
> > Reviewed-on: https://chromium-review.googlesource.com/973267
> > Reviewed-by: Robert Phillips <robertphillips@google.com>
> > Commit-Queue: Brian Salomon <bsalomon@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#544777}
> 
> TBR=bsalomon@chromium.org,robertphillips@google.com
> 
> # Not skipping CQ checks because original CL landed > 1 day ago.
> 
> Change-Id: Icaee5a1afa55bc8aec4567687661dacef0ccf565
> Reviewed-on: https://chromium-review.googlesource.com/980292
> Reviewed-by: Robert Phillips <robertphillips@google.com>
> Commit-Queue: Robert Phillips <robertphillips@google.com>
> Cr-Commit-Position: refs/heads/master@{#545787}

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

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

Bug: 825594, 825585
Change-Id: I82434eebb65257cf6a92225c2c862fe22aecf093
Reviewed-on: https://chromium-review.googlesource.com/1014140Reviewed-by: Brian Salomon <bsalomon@chromium.org>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551022}

skia/config/SkUserConfig.h

@@ -208,11 +208,6 @@ 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