GCC: Add missing default return value for QueryCancellationTraitsImpl()

Without default return value GCC produces a lot of warnings like:
../../base/bind_internal.h:773:1: warning:
control reaches end of non-void function [-Wreturn-type]

Added return false by default to suppress the warnings.

Bug: 819294
Change-Id: I593cd48bf336cb5e7614592a507451a67e0ec1d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2520819
Commit-Queue: José Dapena Paz <jdapena@igalia.com>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825004}

base/bind_internal.h

@@ -770,6 +770,7 @@ bool QueryCancellationTraitsImpl(BindStateBase::CancellationQueryMode mode,
           functor, std::get<indices>(bound_args)...);
   }
   NOTREACHED();
+  return false;
 }
 
 // Relays |base| to corresponding CallbackCancellationTraits<>::Run(). Returns