Created a helper function "FindDiskBySourcePath" in DiskMountManager.

chrome/browser/chromeos/disks/mock_disk_mount_manager.cc

Index: chrome/browser/chromeos/disks/mock_disk_mount_manager.cc
diff --git a/chrome/browser/chromeos/disks/mock_disk_mount_manager.cc b/chrome/browser/chromeos/disks/mock_disk_mount_manager.cc
index 38909657826ee68caf25758e787c844d03f82fc9..de08ebed8e0311b8d95bb1c59bcc6e8e04d1c272 100644
--- a/chrome/browser/chromeos/disks/mock_disk_mount_manager.cc
+++ b/chrome/browser/chromeos/disks/mock_disk_mount_manager.cc
@@ -53,6 +53,9 @@ MockDiskMountManager::MockDiskMountManager() {
       .WillByDefault(Invoke(this, &MockDiskMountManager::disksInternal));
   ON_CALL(*this, mount_points())
       .WillByDefault(Invoke(this, &MockDiskMountManager::mountPointsInternal));
+  ON_CALL(*this, FindDiskBySourcePath(_))
+      .WillByDefault(Invoke(
+          this, &MockDiskMountManager::FindDiskBySourcePathInternal));
 }
 
 MockDiskMountManager::~MockDiskMountManager() {
@@ -147,6 +150,8 @@ void MockDiskMountManager::SetupDefaultReplies() {
       .WillRepeatedly(ReturnRef(disks_));
   EXPECT_CALL(*this, mount_points())
       .WillRepeatedly(ReturnRef(mount_points_));
+  EXPECT_CALL(*this, FindDiskBySourcePath(_))
+      .Times(AnyNumber());
   EXPECT_CALL(*this, RequestMountInfoRefresh())
       .Times(AnyNumber());
   EXPECT_CALL(*this, MountPath(_, _, _, _))
@@ -202,6 +207,13 @@ MockDiskMountManager::mountPointsInternal() const {
   return mount_points_;
 }
 
+const DiskMountManager::Disk*
+MockDiskMountManager::FindDiskBySourcePathInternal(
+    const std::string& source_path) const {
+  DiskMap::const_iterator disk_it = disks_.find(source_path);
+  return disk_it == disks_.end() ? NULL : disk_it->second;
+}
+
 void MockDiskMountManager::NotifyDiskChanged(
     DiskMountManagerEventType event,
     const DiskMountManager::Disk* disk) {