Revert of [wasm] Enable WASM by default (--expose-wasm=true). (patchset #2...

Revert of [wasm] Enable WASM by default (--expose-wasm=true). (patchset #2 id:20001 of https://codereview.chromium.org/2623743003/ )

Reason for revert:
arm64 gc stress:
https://build.chromium.org/p/client.v8.ports/builders/V8%20Linux%20-%20arm64%20-%20sim%20-%20gc%20stress/builds/4424

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-Commit-Position: refs/heads/master@{#42197}
> Committed: https://chromium.googlesource.com/v8/v8/+/34b63f050b1a247bb64ddc91c967501ce04e011f

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/2626763002
Cr-Commit-Position: refs/heads/master@{#42199}

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

@@ -2,4 +2,5 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-assertEquals("function", typeof WebAssembly.Module);
+// TODO(titzer): remove this test when WASM ships.
+assertThrows(function() { var g = Wasm; });