[wasm] Reduce wasm multiple memory test to 20 memories.

600 memories puts a high burden on implementions,
reducing for now to 20.

R=eholk@chromium.org

Bug: 
Change-Id: If2e9b9b0e550ade8a05b036d539d0ac01b2142de
Reviewed-on: https://chromium-review.googlesource.com/842845Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526080}

third_party/WebKit/LayoutTests/external/wpt/wasm/many-memories.window.js

@@ -3,11 +3,11 @@
 // found in the LICENSE file.
 
 // This test makes sure browsers behave reasonably when asked to allocate a
-// large number of WebAssembly.Memory objects at once.
+// larger number of WebAssembly.Memory objects at once.
 test(function() {
   let memories = [];
   try {
-    for (let i = 0; i < 600; i++) {
+    for (let i = 0; i < 20; i++) {
       memories.push(new WebAssembly.Memory({initial: 1}));
     }
   } catch (e) {