cc/paint: Add a paint op helper with a ToString functionality.

This patch adds a helper class that can stringify PaintOps. This is
done in a best effort manner to help with debugging. Also adds
corresponding unittests.

R=enne@chromium.org

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ic5003365251fa40710e7a79ccf3934096d9f7d6a
Reviewed-on: https://chromium-review.googlesource.com/764390
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516057}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 83dba38..3a96df7 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -461,6 +461,7 @@
     "test/mock_occlusion_tracker.h",
     "test/ordered_texture_map.cc",
     "test/ordered_texture_map.h",
+    "test/paint_op_helper.h",
     "test/pixel_comparator.cc",
     "test/pixel_comparator.h",
     "test/pixel_test.cc",
@@ -619,6 +620,7 @@
     "paint/oop_pixeltest.cc",
     "paint/paint_image_unittest.cc",
     "paint/paint_op_buffer_unittest.cc",
+    "paint/paint_op_helper_unittest.cc",
     "paint/paint_shader_unittest.cc",
     "paint/scoped_image_flags_unittest.cc",
     "paint/solid_color_analyzer_unittest.cc",