[piexwasm] Reset ImageBuffer .memory property on close

On releasing our allocated .memory, reset this.memory back to 0. Minor
closure fix: remove @struct from PixeLoader, since that class does not
have any |this| properties by-design.

Bug: 1132695
No-try: true
Change-Id: Icb2d9f7cf6fcfdb0d89c2dd1c69c89958987fbc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437316
Commit-Queue: Noel Gordon <noel@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Alex Danilo <adanilo@chromium.org>
Auto-Submit: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811958}

ui/file_manager/image_loader/piex_loader.js

@@ -250,8 +250,8 @@ class ImageBuffer {
   /**
    * Calls Module.image() to process |this.source| and return the result.
    *
+   * @throws {!Error} Memory allocation error.
    * @return {!PiexWasmImageResult}
-   * @throws {!Error}
    */
   process() {
     this.memory = PiexModule._malloc(this.length);
@@ -474,14 +474,17 @@ class ImageBuffer {
    * Release resources.
    */
   close() {
-    PiexModule._free(this.memory);
+    const memory = this.memory;
+    if (memory) {
+      PiexModule._free(memory);
+      this.memory = 0;
+    }
   }
 }
 
 /**
  * Creates a PiexLoader.
  * @constructor
- * @struct
  */
 function PiexLoader() {}