Revert of [wasm] Enable WASM by default (--expose-wasm=true). (patchset #3...

Revert of [wasm] Enable WASM by default (--expose-wasm=true). (patchset #3 id:40001 of https://codereview.chromium.org/2623743003/ )

Reason for revert:
layout test breakages:
https://build.chromium.org/p/client.v8.fyi/builders/V8-Blink%20Linux%2064/builds/12574

revert first to unblock rolling. otherwise please rebaseline.

Original issue's description:
> [wasm] Enable WASM by default (--expose-wasm=true).
>
> BUG=chromium:575167
> R=rossberg@chromium.org,ahaas@chromium.org,clemensh@chromium.org,bradnelson@chromium.org
>
> Review-Url: https://codereview.chromium.org/2623743003
> Cr-Original-Commit-Position: refs/heads/master@{#42197}
> Committed: https://chromium.googlesource.com/v8/v8/+/34b63f050b1a247bb64ddc91c967501ce04e011f
> Review-Url: https://codereview.chromium.org/2623743003
> Cr-Commit-Position: refs/heads/master@{#42214}
> Committed: https://chromium.googlesource.com/v8/v8/+/71f5650828ffd0162fcd67d9c6ef570a346a8a84

TBR=clemensh@chromium.org,ahaas@chromium.org,bradnelson@chromium.org,rossberg@chromium.org,bradnelson@google.com,titzer@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:575167

Review-Url: https://codereview.chromium.org/2621323002
Cr-Commit-Position: refs/heads/master@{#42222}

src/flag-definitions.h

@@ -497,7 +497,7 @@ DEFINE_NEG_IMPLICATION(minimal, crankshaft)
 DEFINE_NEG_IMPLICATION(minimal, use_ic)
 
 // Flags for native WebAssembly.
-DEFINE_BOOL(expose_wasm, true, "expose WASM interface to JavaScript")
+DEFINE_BOOL(expose_wasm, false, "expose WASM interface to JavaScript")
 DEFINE_INT(wasm_num_compilation_tasks, 10,
            "number of parallel compilation tasks for wasm")
 DEFINE_BOOL(trace_wasm_encoder, false, "trace encoding of wasm code")

test/mjsunit/mjsunit.status

@@ -240,9 +240,6 @@
 
   #BUG(v8:5683)
   'wasm/import-memory': [SKIP],
-
-  # Crashes with WASM turned on by default on arm64 gc-stress
-  'regress/regress-500831': [PASS, ['arch == arm64', SKIP]],
 }],  # 'gc_stress == True'
 
 ##############################################################################

test/mjsunit/wasm/wasm-default.js → test/mjsunit/wasm/no-wasm-by-default.js

@@ -2,18 +2,5 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-function GetWebAssembly() {
-  return WebAssembly;
-}
-
-let WASM_ON_BY_DEFAULT = true;
-
-if (WASM_ON_BY_DEFAULT) {
-  try {
-    assertFalse(undefined === GetWebAssembly());
-  } catch (e) {
-    assertTrue(false);
-  }
-} else {
-  assertThrows(GetWebAssembly);
-}
+// TODO(titzer): remove this test when WASM ships.
+assertThrows(function() { var g = Wasm; });