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

Since native_client_sdk can't include base/ headers, and since
the sdk is always built by clang, just use the clang attribute
directly. (Alternatively, we could've disabled the warning for
this target.)

This CL was uploaded by git cl split.

R=binji@chromium.org

Bug: 177475
Change-Id: I14cf95478a0da89a8426e748ded165eb177c4c24
Reviewed-on: https://chromium-review.googlesource.com/891900Reviewed-by: Ben Smith <binji@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533328}

native_client_sdk/src/libraries/nacl_io/httpfs/http_fs_node.cc

@@ -62,7 +62,8 @@ StringMap_t ParseHeaders(const char* headers, int32_t headers_length) {
         // Found a non-whitespace, mark this as the start of the value.
         start = &headers[i];
         state = FINDING_VALUE;
-      // Fallthrough to start processing value without incrementing i.
+        // Fallthrough to start processing value without incrementing i.
+        [[clang::fallthrough]];
 
       case FINDING_VALUE:
         if (headers[i] == '\n') {