[piexwasm] Wasm is a 32-bit system: restrict image length

Wasm is a 32-bit system, and hence uses 32-bit integers. Restrict the
input image length (a JS int) to << 32-bits (2^30 for now).

Bug: 935285
Change-Id: I2d523e49b2392fb49f8b25e9b77e8334532b5b5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1546803Reviewed-by: Alex Danilo <adanilo@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646606}

ui/file_manager/image_loader/piex/tests.html

@@ -27,6 +27,9 @@ piex wasm raw image preview / thumbnail test page
     }
 
     process(resolve, reject) {
+      if (this.length <= 0 || this.length >= Math.pow(2, 30))
+        return reject(new Error('invalid image length'));
+ 
       const memory = Module._malloc(this.length);
       if (!memory)
         return reject(new Error('image malloc failure'));