Fixes for re-enabling more MSVC level 4 warnings: misc edition #3

This contains fixes for the following sorts of issues:
* Signedness mismatch
* Possibly-uninitialized local variable

These are the warnings that have appeared in the codebase since I began
uploading my first cleanup pass a few weeks ago.

BUG=81439
TEST=none

Review URL: https://codereview.chromium.org/400313002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284378 0039d316-1c4b-4281-b951-d872f2087c98

cc/surfaces/surface_aggregator_unittest.cc

@@ -31,7 +31,7 @@ namespace {
 
 SurfaceId InvalidSurfaceId() {
   static SurfaceId invalid;
-  invalid.id = -1;
+  invalid.id = static_cast<uint64_t>(-1);
   return invalid;
 }
 

chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.cc

@@ -362,13 +362,12 @@ ParentIDAndTitle MetadataDatabaseIndexOnDisk::PickMultiBackingFilePath() const {
     return ParentIDAndTitle();
 
   size_t pos = value.find('\0');  // '\0' is a separator.
-  int64 parent_id;
-  if (pos == std::string::npos ||
-      !base::StringToInt64(value.substr(0, pos), &parent_id))
+  if (pos == std::string::npos)
     return ParentIDAndTitle();
 
-  // Successfully found an entry.
-  return ParentIDAndTitle(parent_id, value.substr(pos + 1));
+  int64 parent_id;
+  return base::StringToInt64(value.substr(0, pos), &parent_id) ?
+      ParentIDAndTitle(parent_id, value.substr(pos + 1)) : ParentIDAndTitle();
 }
 
 int64 MetadataDatabaseIndexOnDisk::PickDirtyTracker() const {