Streams: Add commented-out assert() impl

It's a waste of time reimplementing assert() every time you need it. Instead,
include a definition of assert() in CommonOperations.js. The implementation is
commented-out so it will have no runtime overhead (comments are not included in
the V8 snapshot).

https://bugs.chromium.org/p/chromium/issues/detail?id=662542 aims to find
the long-term solution to making asserts work. This is just a stop-gap to
make them less painful in the short term.

Bug: 
Change-Id: Ib62851d19183b805f164a281c2f5c79a33d92c4b
Reviewed-on: https://chromium-review.googlesource.com/594127Reviewed-by: Takeshi Yoshino <tyoshino@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491324}

third_party/WebKit/Source/core/streams/CommonOperations.js

@@ -34,6 +34,23 @@
     return Boolean(x) && hasOwnProperty(x, property);
   }
 
+  //
+  // Assert is not normally enabled, to avoid the space and time overhead. To
+  // enable, uncomment this definition and then in the file you wish to enable
+  // asserts for, uncomment the assert statements and add this definition:
+  // const assert = pred => binding.SimpleAssert(pred);
+  //
+  // binding.SimpleAssert = pred => {
+  //   if (pred) {
+  //     return;
+  //   }
+  //   v8.log('\n\n\n  *** ASSERTION FAILURE ***\n\n');
+  //   v8.logStackTrace();
+  //   v8.log('**************************************************\n\n');
+  //   class StreamsAssertionError extends Error {}
+  //   throw new StreamsAssertionError('Streams Assertion Failure');
+  // }
+
   //
   // Promise-manipulation functions
   //