cc: ImageDecodes: Skip allocating a temporary buffer for subrect pixels
cc: ImageDecodes: Skip allocating a temporary buffer for subrect pixels

This patch uses SkPixmap::extractSubset instead of reading pixels into
a temporary buffer. Since this is all software pixels, extractSubset
does some pointer math and adjusts SkDrawInfo to extract the subset
basically for free.

R=enne, ericrk
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1757383002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#379119}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk