Make --future imply --concurrent_compiler_frontend.

This feature is still far from doing what its name suggests, but we want
to get test coverage.

R=hablich@chromium.org, jarin@chromium.org

Bug: v8:7790
Change-Id: I93c5c312e3dc84d04eb311ac17e078dc940471c8
Reviewed-on: https://chromium-review.googlesource.com/1185095Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55319}

src/flag-definitions.h

@@ -389,6 +389,7 @@ DEFINE_BOOL(block_concurrent_recompilation, false,
             "block queued jobs until released")
 DEFINE_BOOL(concurrent_compiler_frontend, false,
             "run optimizing compiler's frontend phases on a separate thread")
+DEFINE_IMPLICATION(future, concurrent_compiler_frontend)
 DEFINE_BOOL(strict_heap_broker, false, "fail on incomplete serialization")
 DEFINE_BOOL(trace_heap_broker, false, "trace the heap broker")