Remove fallthrough in http_fs_node.cc

Fallthrough in a switch statement now omits a warning in modern
compilers. Unfortunately, the NaCl compilers are too old to support
the required annotation. We could enable this conditionally with a
macro, but it's simpler to just remove the fallthrough and replace
it with a goto.

Change-Id: I0c112883d170155424c8c40879d3024e2d144c8e
Reviewed-on: https://chromium-review.googlesource.com/965563Reviewed-by: Sam Clegg <sbc@chromium.org>
Commit-Queue: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544124}

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

@@ -62,9 +62,14 @@ 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;
+
+        // NOTE: Avoid fallthrough as it produces a warning on newer compilers,
+        // but can't easily be silenced by the older NaCl compilers.
+        //
         // Fallthrough to start processing value without incrementing i.
-        [[clang::fallthrough]];
+        goto finding_value;
 
+      finding_value:
       case FINDING_VALUE:
         if (headers[i] == '\n') {
           // Found value.