[Flags] Only enable Liftoff on x86 and x64

So far, v8 only implemented Liftoff on x86 and x64, and enabling it on
other platforms basically did not have any effect. Now we start porting
to arm. Until this is complete, we do not want this to be used in the wild.

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

Bug: chromium:787421
Change-Id: Ie7978dc579cbc9203647ea5395302cda57fc0403
Reviewed-on: https://chromium-review.googlesource.com/c/1331388Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608303}

content/public/common/content_features.cc

@@ -464,7 +464,12 @@ const base::Feature kWebAssembly{"WebAssembly",
 
 // Enable WebAssembly baseline compilation and tier up.
 const base::Feature kWebAssemblyBaseline{"WebAssemblyBaseline",
-                                         base::FEATURE_ENABLED_BY_DEFAULT};
+#ifdef ARCH_CPU_X86_FAMILY
+                                         base::FEATURE_ENABLED_BY_DEFAULT
+#else
+                                         base::FEATURE_DISABLED_BY_DEFAULT
+#endif
+};
 
 // Enable WebAssembly threads.
 // https://github.com/WebAssembly/threads