Make TaskGraphWorkQueue more robust against changing dependencies

Currently, if a task in the TaskGraphWorkQueue goes from having no
dependencies to having dependencies, the TaskGraphWorkQueue may
incorrectly schedule it to run twice.

This change adds an additional check to make sure we handle this case.

R=vmpstr
BUG=652718
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2643153002
Cr-Commit-Position: refs/heads/master@{#445425}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 3f37b8b..911fe486 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -813,6 +813,7 @@
     "raster/scoped_gpu_raster_unittest.cc",
     "raster/single_thread_task_graph_runner_unittest.cc",
     "raster/synchronous_task_graph_runner_unittest.cc",
+    "raster/task_graph_work_queue_unittest.cc",
     "raster/texture_compressor_etc1_unittest.cc",
     "resources/platform_color_unittest.cc",
     "resources/resource_pool_unittest.cc",