Remove flaky webview-only cc_unittest that shouldnt run single-thread

LayerTreeHostFreesWorkerContextResourcesOnZeroMemoryLimitSynchronous
test was set to run with SINGLE_AND_MULTI_THREAD_TEST_F but since it
is testing webview-only things, this caused DCHECKs to fail, in single-
threaded compositor code that does not expect to be called. Disable
the single-threaded version of the test, as WebView only uses/supports
multi-threaded cc.

R=enne@chromium.org

Bug: 778404
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ie4076509ed550977acd61f1958165636e6265122
Reviewed-on: https://chromium-review.googlesource.com/1028186
Commit-Queue: enne <enne@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554096}

cc/trees/layer_tree_host_unittest.cc

@@ -563,7 +563,8 @@ class LayerTreeHostFreesWorkerContextResourcesOnZeroMemoryLimitSynchronous
   }
 };
 
-SINGLE_AND_MULTI_THREAD_TEST_F(
+// Android Webview only runs in multi-threaded compositing mode.
+MULTI_THREAD_TEST_F(
     LayerTreeHostFreesWorkerContextResourcesOnZeroMemoryLimitSynchronous);
 
 // Test if the LTH successfully frees main and worker resources when the

cc/trees/single_thread_proxy.cc

@@ -839,7 +839,9 @@ void SingleThreadProxy::ScheduledActionPrepareTiles() {
 }
 
 void SingleThreadProxy::ScheduledActionInvalidateLayerTreeFrameSink() {
-  NOTREACHED();
+  // This is an Android WebView codepath, which only uses multi-thread
+  // compositor. So this should not occur in single-thread mode.
+  NOTREACHED() << "Android Webview use-case, so multi-thread only";
 }
 
 void SingleThreadProxy::ScheduledActionPerformImplSideInvalidation() {