[test] Remove eval string confusing the fuzzer.

This removes an occurrence where the "%Foo" native syntax appears as part
of a string. Such strings are picked up by the fuzzer and recombined in
unsupported ways, producing false-positive crash reports. Simply avoid
having those strings in the fuzzing corpus.

R=clemensh@chromium.org
TEST=mjsunit/regress/wasm/regress-808848
BUG=chromium:844842

Change-Id: I017c1552578f0d26033e58b11353e87e27a69ebf
Reviewed-on: https://chromium-review.googlesource.com/1136300Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54432}

test/mjsunit/regress/wasm/regress-808848.js

@@ -49,16 +49,16 @@ let m1 = new WebAssembly.Module(m1_bytes);
 // Serialize the module and postMessage it to another thread.
 let serialized_m1 = %SerializeWasmModule(m1);
 
-let workerScript =
-  `onmessage = function(msg) {
-    let {serialized_m1, m1_bytes} = msg;
+let worker_onmessage = function(msg) {
+  let {serialized_m1, m1_bytes} = msg;
 
-    let m1_clone = %DeserializeWasmModule(serialized_m1, m1_bytes);
-    let imports = {mod: {get: () => 3, call: () => {}}};
-    let i2 = new WebAssembly.Instance(m1_clone, imports);
-    i2.exports.main();
-    postMessage('done');
-  }`;
+  let m1_clone = %DeserializeWasmModule(serialized_m1, m1_bytes);
+  let imports = {mod: {get: () => 3, call: () => {}}};
+  let i2 = new WebAssembly.Instance(m1_clone, imports);
+  i2.exports.main();
+  postMessage('done');
+}
+let workerScript = "onmessage = " + worker_onmessage.toString();
 
 let worker = new Worker(workerScript);
 worker.postMessage({serialized_m1, m1_bytes});