Explicitly suppress -Wimplicit-fallthrough for v8.

No behavior change since that warning isn't enabled anywhere yet, and
it's opt-in.  This is a prerequisite for
https://chromium-review.googlesource.com/c/chromium/src/+/895726

Whil here, also remove -Wsign-compare, since that's enabled by default for -Wextra
(http://llvm-cs.pcc.me.uk/tools/clang/include/clang/Basic/DiagnosticGroups.td#723)
and v8 builds with the chromium_code config which sets -Wextra.

Bug: chromium:177475
Change-Id: I32f140296cab13ed57be08168a3f6b1d1218e4e6
Reviewed-on: https://chromium-review.googlesource.com/895704Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51009}

BUILD.gn

@@ -520,10 +520,11 @@ config("toolchain") {
 
   if (is_clang) {
     cflags += [
-      "-Wsign-compare",
-
       # 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") {