v8: Turn on -Wimplicit-fallthrough.

https://chromium-review.googlesource.com/911731 made things build with
this on x64, and https://chromium-review.googlesource.com/923528 added
x86, arm, arm64, mips, mips64.  This are all the configs covered by
v8's trybots.  If this breaks yet another config I don't know about,
these two CLs should give you a good idea how to fix them.

Bug: chromium:812686
Change-Id: Ib9a9714a070dd876a8f5911a1bc974ffd7aa3995
Reviewed-on: https://chromium-review.googlesource.com/928842
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51446}

BUILD.gn

@@ -529,9 +529,6 @@ config("toolchain") {
     cflags += [
       # TODO(hans): Remove once http://crbug.com/428099 is resolved.
       "-Winconsistent-missing-override",
-
-      # TODO(thakis): Consider enabling this, https://crbug.com/807632
-      "-Wno-implicit-fallthrough",
     ]
 
     if (v8_current_cpu != "mips" && v8_current_cpu != "mipsel") {