Split out web layer tests from web_frame_test.cc to web_layer_test.cc

web_frame_test.cc is huge! This patch splits out the blink->compositor
tests into web_layer_test.cc. This is primarily a code move but the
following renames have also been included:
  SlimmingPaintWebFrameTest -> WebLayerListTest
  SlimmingPaintWebFrameSimTest -> WebLayerListSimTest

Change-Id: I78d2dbd242c1a69dbbc9a76dcf2be20087018508
Reviewed-on: https://chromium-review.googlesource.com/c/1327152
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606648}

third_party/blink/renderer/core/BUILD.gn

@@ -1839,6 +1839,7 @@ jumbo_source_set("unit_tests") {
     "exported/web_frame_serializer_sanitization_test.cc",
     "exported/web_frame_serializer_test.cc",
     "exported/web_frame_test.cc",
+    "exported/web_layer_test.cc",
     "exported/web_meaningful_layouts_test.cc",
     "exported/web_node_test.cc",
     "exported/web_plugin_container_test.cc",