Work more on getting things built with -Wimplicit-fallthrough.

This CL was uploaded by git cl split.

R=kouhei@chromium.org

Bug: 177475
Change-Id: I09a969c86b5250e6104caf8b0d7ba1672425a8d3
Reviewed-on: https://chromium-review.googlesource.com/889945
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532338}

third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp

@@ -1828,7 +1828,7 @@ std::unique_ptr<TextResourceDecoder> XMLHttpRequest::CreateDecoder() const {
     case kResponseTypeDefault:
       if (ResponseIsXML())
         return TextResourceDecoder::Create(decoder_options_for_xml);
-    // fall through
+      FALLTHROUGH;
     case kResponseTypeText:
       return TextResourceDecoder::Create(decoder_options_for_utf8_plain_text);
     case kResponseTypeDocument: