Fix MediaStorageUtil::CrackDeviceId function to include ":" in the media device type prefix.

chrome/browser/media_gallery/media_storage_util.cc

Index: chrome/browser/media_gallery/media_storage_util.cc
diff --git a/chrome/browser/media_gallery/media_storage_util.cc b/chrome/browser/media_gallery/media_storage_util.cc
index 2379d70d3d98b573ea9e08a7f0cf4e146143b1d4..0242690e94d55566d9d7f5e10caa066b36a7df39 100644
--- a/chrome/browser/media_gallery/media_storage_util.cc
+++ b/chrome/browser/media_gallery/media_storage_util.cc
@@ -101,7 +101,8 @@ std::string MediaStorageUtil::MakeDeviceId(Type type,
 bool MediaStorageUtil::CrackDeviceId(const std::string& device_id,
                                      Type* type, std::string* unique_id) {
   size_t prefix_length = device_id.find_first_of(':');
-  std::string prefix = device_id.substr(0, prefix_length);
+  std::string prefix = prefix_length != std::string::npos ?
+                       device_id.substr(0, prefix_length + 1) : "";
 
   Type found_type;
   if (prefix == kUsbMassStorageWithDCIMPrefix) {