Fix GCC 5.4 compilation error

GCC 5.4 complains type mismatch as follows:

../../v8/src/contexts.cc:302:34: error: format '%hhu' expects
argument of type 'int', but argument 3 has type
'v8::internal::VariableMode' [-Werror=format=]

Bug: chromium:841460
Change-Id: Id90c1211f459309e477a8ad5658cecdf9cc10938
Reviewed-on: https://chromium-review.googlesource.com/1077051Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#53443}

src/contexts.cc

@@ -299,7 +299,7 @@ Handle<Object> Context::Lookup(Handle<String> name, ContextLookupFlags flags,
       if (slot_index >= 0) {
         if (FLAG_trace_contexts) {
           PrintF("=> found local in context slot %d (mode = %hhu)\n",
-                 slot_index, mode);
+                 slot_index, static_cast<uint8_t>(mode));
         }
         *index = slot_index;
         *variable_mode = mode;