MediaGalleriesCustomBindings: Ensure blobs aren't GC'd when getMetadata runs.

BUG=415792

Review URL: https://codereview.chromium.org/615633003

Cr-Commit-Position: refs/heads/master@{#297444}

chrome/renderer/resources/extensions/OWNERS

@@ -20,6 +20,7 @@ per-file web_view*.js=fsamuel@chromium.org
 per-file web_view*.js=lazyboy@chromium.org
 per-file app_view*.js=fsamuel@chromium.org
 per-file app_view*.js=lazyboy@chromium.org
+per-file media_galleries*.js=orenb@chromium.org
 per-file enterprise_platform_keys*=pneubeck@chromium.org
 per-file notifications_*.js=dewittj@chromium.org
 per-file image_util.js=dewittj@chromium.org

chrome/renderer/resources/extensions/media_galleries_custom_bindings.js

@@ -8,6 +8,7 @@ var binding = require('binding').Binding.create('mediaGalleries');
 var blobNatives = requireNative('blob_natives');
 var mediaGalleriesNatives = requireNative('mediaGalleries');
 
+var blobsAwaitingMetadata = {};
 var mediaGalleriesMetadata = {};
 
 function createFileSystemObjectsAndUpdateMetadata(response) {
@@ -101,6 +102,10 @@ binding.registerCustomHook(function(bindingsAPI, extensionId) {
   apiFunctions.setUpdateArgumentsPostValidate('getMetadata',
       function(mediaFile, options, callback) {
     var blobUuid = blobNatives.GetBlobUuid(mediaFile)
+    // Store the blob in a global object to keep its refcount nonzero -- this
+    // prevents the object from being garbage collected before any metadata
+    // parsing gets to occur (see crbug.com/415792).
+    blobsAwaitingMetadata[blobUuid] = mediaFile;
     return [blobUuid, options, callback];
   });
 
@@ -118,6 +123,11 @@ binding.registerCustomHook(function(bindingsAPI, extensionId) {
     if (request.callback)
       request.callback(response.metadata);
     request.callback = null;
+
+    // The UUID was in position 0 in the setUpdateArgumentsPostValidate
+    // function.
+    var uuid = request.args[0];
+    delete blobsAwaitingMetadata[uuid];
   });
 });