Remaining changes to build all code in src.git with -Wimplicit-fallthrough.

This CL was uploaded by git cl split.

R=bauerb@chromium.org

Bug: 177475
Change-Id: Idb94f3a156bf2855422b1f140ef7e2ae5598b60d
Reviewed-on: https://chromium-review.googlesource.com/891923Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532879}

chrome/browser/ui/webui/theme_source.cc

@@ -117,6 +117,7 @@ void ThemeSource::StartDataRequest(
       case version_info::Channel::BETA:
       case version_info::Channel::STABLE:
         NOTREACHED();
+        FALLTHROUGH;
 #endif
       case version_info::Channel::UNKNOWN:
         resource_id = IDR_PRODUCT_LOGO_32;