cc: Add tests for DisplayListRasterSource.

This mimics (most of) the tests from PicturePileImpl but for the
display list backend to verify gather pixel refs and raster
behaviour. Also adds a new partial raster test.

This gives me a framework to write unit tests for culling display
items.

R=enne, vmpstr
BUG=487387
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1146813012

Cr-Commit-Position: refs/heads/master@{#332676}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index dfe9f121..58540a9 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -829,6 +829,7 @@
     "output/software_renderer_unittest.cc",
     "output/texture_mailbox_deleter_unittest.cc",
     "playback/display_item_list_unittest.cc",
+    "playback/display_list_raster_source_unittest.cc",
     "playback/display_list_recording_source_unittest.cc",
     "playback/picture_pile_impl_unittest.cc",
     "playback/picture_pile_unittest.cc",