[piexwasm] Add piex loader readFromFileSystem helper

Raw image preview requests contain the source file as a DOM fileSystem
URL. Add a helper to resolve the fileEntry associated with the URL and
read its content into an ArrayBuffer with the DOM fileSystem API.

Add closure markup / comments, add externs/platform.js to BUILD.gn for
the DOM fileSystem API declarations, and Closure compile the lot.

Bug: 904630
Change-Id: I80d3f695dfce939b92d42932db666c3ab272ac69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611624Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660692}

ui/file_manager/image_loader/BUILD.gn

@@ -94,7 +94,10 @@ js_library("piex_loader") {
     ":load_image_request",
     "../file_manager/foreground/js/metadata:image_orientation",
   ]
-  externs_list = [ "$externs_path/chrome_extensions.js" ]
+  externs_list = [
+    "$externs_path/chrome_extensions.js",
+    "../externs/platform.js",
+  ]
 }
 
 js_unittest("piex_loader_unittest") {

ui/file_manager/image_loader/piex_loader.js

@@ -381,6 +381,53 @@ PiexLoader.prototype.simulateIdleTimeoutPassedForTests = function() {
   }
 };
 
+/**
+ * Resolves the file entry associated with DOM filesystem |url| and returns
+ * the file content in an ArrayBuffer.
+ * @param {string} url - DOM filesystem URL of the file.
+ * @returns {!Promise<!ArrayBuffer>}
+ */
+function readFromFileSystem(url) {
+  return new Promise((resolve, reject) => {
+    /**
+     * Reject the Promise on fileEntry URL resolve or file read failures.
+     */
+    function failure(error) {
+      reject(new Error('Reading file system: ' + error));
+    }
+
+    /**
+     * Returns true if the fileEntry file size is within sensible limits.
+     * @param {number} size - file size.
+     * @return {boolean}
+     */
+    function valid(size) {
+      return size > 0 && size < Math.pow(2, 30);
+    }
+
+    /**
+     * Reads the fileEntry's content into an ArrayBuffer: resolve Promise
+     * with the ArrayBuffer result or reject the Promise on failure.
+     * @param {!Entry} entry - file system entry of |url|.
+     */
+    function readEntry(entry) {
+      const fileEntry = /** @type {!FileEntry} */ (entry);
+      fileEntry.file((file) => {
+        if (valid(file.size)) {
+          const reader = new FileReader();
+          reader.onerror = failure;
+          reader.onload = (_) => resolve(reader.result);
+          reader.readAsArrayBuffer(file);
+        } else {
+          failure('invalid file size: ' + file.size);
+        }
+      }, failure);
+    }
+
+    window.webkitResolveLocalFileSystemURL(url, readEntry, failure);
+  });
+}
+
 /**
  * Starts to load RAW image.
  * @param {string} url