Benchmark
out\release\libyuv_unittest --libyuv_width=1279 --libyuv_height=719 --libyuv_repeat=999 --libyuv_flags=-1 --gtest_filter=*Blend* | sortms

Was
I420Blend_Any (2321 ms)
I420Blend_Unaligned (1684 ms)
I420Blend_Opt (1675 ms)
I420Blend_Invert (1653 ms)
BlendPlane_Invert (1556 ms)
BlendPlane_Any (1552 ms)
BlendPlane_Unaligned (1548 ms)
BlendPlane_Opt (1535 ms)
ARGBBlend_Unaligned (659 ms)
ARGBBlend_Any (596 ms)
ARGBBlend_Invert (591 ms)
ARGBBlend_Opt (508 ms)
BlendPlaneRow_Unaligned (186 ms)
BlendPlaneRow_Opt (171 ms)

Now
ARGBBlend_Any (621 ms)
ARGBBlend_Unaligned (585 ms)
ARGBBlend_Invert (564 ms)
ARGBBlend_Opt (512 ms)
I420Blend_Unaligned (347 ms)
I420Blend_Invert (345 ms)
I420Blend_Any (337 ms)
I420Blend_Opt (327 ms)
BlendPlane_Unaligned (187 ms)
BlendPlaneRow_Unaligned (187 ms)
BlendPlane_Invert (186 ms)
BlendPlane_Any (186 ms)
BlendPlaneRow_Opt (173 ms)
BlendPlane_Opt (171 ms)

which is comparable to aligned case
out\release\libyuv_unittest --libyuv_width=1280 --libyuv_height=720 --libyuv_repeat=999 --libyuv_flags=-1 --gtest_filter=*Blend* | sortms
ARGBBlend_Any (625 ms)
ARGBBlend_Unaligned (602 ms)
ARGBBlend_Invert (508 ms)
ARGBBlend_Opt (506 ms)
I420Blend_Any (353 ms)
I420Blend_Unaligned (322 ms)
I420Blend_Invert (304 ms)
I420Blend_Opt (301 ms)
BlendPlaneRow_Unaligned (188 ms)
BlendPlane_Unaligned (186 ms)
BlendPlane_Invert (185 ms)
BlendPlane_Any (184 ms)
BlendPlaneRow_Opt (173 ms)
BlendPlane_Opt (169 ms)

R=dhrosa@google.com, harryjin@google.com
BUG=libyuv:527

Review URL: https://codereview.chromium.org/1513443002 .