[Flags] Explicitly disable Liftoff if tier up is disabled

In V8, we still have an ordering problem with processing flags. In this
case, the "--wasm-tier-up" flag implies "--liftoff", and since
"--wasm-tier-up" is enabled by default, also "--liftoff" is enabled by
default. If we now set "--no-wasm-tier-up" (via {SetFlagsFromString}),
this does not disable "--liftoff". Hence, disable it explicitly.

R=jochen@chromium.org, titzer@chromium.org

Bug: chromium:866924, chromium:787421
Change-Id: I298792bda07ef02a8f039c72be165ec815baa46f
Reviewed-on: https://chromium-review.googlesource.com/1148386Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577571}

content/renderer/render_process_impl.cc

@@ -144,8 +144,10 @@ RenderProcessImpl::RenderProcessImpl(
   SetV8FlagIfFeature(features::kV8VmFuture, "--future");
   SetV8FlagIfNotFeature(features::kV8VmFuture, "--no-future");
 
-  SetV8FlagIfFeature(features::kWebAssemblyBaseline, "--wasm-tier-up");
-  SetV8FlagIfNotFeature(features::kWebAssemblyBaseline, "--no-wasm-tier-up");
+  SetV8FlagIfFeature(features::kWebAssemblyBaseline,
+                     "--liftoff --wasm-tier-up");
+  SetV8FlagIfNotFeature(features::kWebAssemblyBaseline,
+                        "--no-liftoff --no-wasm-tier-up");
 
   if (base::FeatureList::IsEnabled(features::kWebAssemblyThreads)) {
     constexpr char kFlags[] =