cc: Add PaintImage API for subsetting images.

Add an API to subset a PaintImage, similar to an SkImage. This removes
a common case where the PaintImageGenerator/PaintRecord backing of an
image is lost by being wrapped in a SkImageGenerator backed SkImage.

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Idb529a6932ce330ebd61dea70d2cf0b3c12be6e3
Bug: 735741
Reviewed-on: https://chromium-review.googlesource.com/607211
Commit-Queue: Khushal <khushalsagar@chromium.org>
Reviewed-by: Vladimir Levin <vmpstr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496879}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 8ad7f8a..db4f46f 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -692,6 +692,7 @@
     "output/texture_mailbox_deleter_unittest.cc",
     "paint/discardable_image_map_unittest.cc",
     "paint/display_item_list_unittest.cc",
+    "paint/paint_image_unittest.cc",
     "paint/paint_op_buffer_unittest.cc",
     "paint/solid_color_analyzer_unittest.cc",
     "quads/draw_polygon_unittest.cc",