Add an --expose-wasm flag.

This adds a runtime flag that controls whether the WASM object is
exposed to JavaScript. While currently guarded by the V8_WASM build-time
flag, after landing in V8 the build-time flag will be removed, leaving
only this new runtime flag.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1507623002

Cr-Commit-Position: refs/heads/master@{#32657}

src/bootstrapper.cc

@@ -2657,7 +2657,9 @@ bool Genesis::InstallSpecialObjects(Handle<Context> native_context) {
   }
 
 #if defined(V8_WASM)
+  if (FLAG_expose_wasm) {
     WasmJs::Install(isolate, global);
+  }
 #endif
 
   return true;

src/flag-definitions.h

@@ -465,6 +465,7 @@ DEFINE_BOOL(trace_turbo_escape, false, "enable tracing in escape analysis")
 
 #if defined(V8_WASM)
 // Flags for native WebAssembly.
+DEFINE_BOOL(expose_wasm, true, "expose WASM interface to JavaScript")
 DEFINE_BOOL(trace_wasm_decoder, false, "trace decoding of wasm code")
 DEFINE_BOOL(trace_wasm_decode_time, false, "trace decoding time of wasm code")
 DEFINE_BOOL(trace_wasm_compiler, false, "trace compiling of wasm code")