Remaining tricky bits to build webkit_unit_tests with -Wimplicit-fallthrough.

See https://bugs.chromium.org/p/chromium/issues/detail?id=722928#c72
which says that FALLTHROUGH is intended here.

This CL was uploaded by git cl split.

R=tdresser@chromium.org

Bug: 177475
Change-Id: I3555a1bb32d6b7ec5a2f68dd13266da2e48af62c
Reviewed-on: https://chromium-review.googlesource.com/890429Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532514}

content/renderer/input/frame_input_handler_impl.cc

@@ -431,6 +431,7 @@ FrameInputHandlerImpl::HandlingState::HandlingState(
   switch (state) {
     case UpdateState::kIsPasting:
       render_frame->set_is_pasting(true);
+      FALLTHROUGH;  // Matches RenderFrameImpl::OnPaste() which sets both.
     case UpdateState::kIsSelectingRange:
       render_frame->set_handling_select_range(true);
       break;