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Adds Arm NEON versions of convolve_4_rows_horizontally,
convolve_horizontally and convolve_vertically.

This NEON convolution code was originally added here: [1]. Then, at the
introduction of GPU raster, the code (then in SkBitmapFilter_opts.h) was
removed [2].

The re-introduction of this code gives a significant performance boost
to Arm platforms which do not have GPU raster enabled - currently all
Arm Chromebooks and old Android devices.

Similar SIMD-accelerated convolution code exists in src/skia/ext for
Intel and MIPS platforms which do not have GPU raster enabled. This
commit simply adds an Arm NEON counterpart.

[1] https://codereview.chromium.org/27533004
[2] https://skia-review.googlesource.com/c/skia/+/24644

Change-Id: Ied7d65c0f1e2fd3cb54dee3ff70dcccb8eafcb8a
Reviewed-on: https://chromium-review.googlesource.com/911789
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539143}