[BlobStorage] Fixed reading metrics error value

Bug: 756447, 788681
Change-Id: I6cd4d87384bd55f7168a6c8889c631f2f7d9c015
Reviewed-on: https://chromium-review.googlesource.com/996461Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548320}

third_party/WebKit/Source/core/fileapi/FileReaderLoader.cpp

@@ -301,7 +301,7 @@ void FileReaderLoader::OnComplete(int32_t status, uint64_t data_length) {
   if (status != net::OK || data_length != total_bytes_) {
     net_error_ = status;
     if (net_error_ != net::OK)
-      file_reader_loader_read_errors_histogram.Sample(net_error_);
+      file_reader_loader_read_errors_histogram.Sample(std::max(0, -net_error_));
     Failed(status == net::ERR_FILE_NOT_FOUND ? FileError::kNotFoundErr
                                              : FileError::kNotReadableErr);
     return;

third_party/WebKit/Source/modules/indexeddb/IDBRequestLoader.cpp

@@ -120,7 +120,8 @@ void IDBRequestLoader::DidFail(FileError::ErrorCode) {
   DEFINE_THREAD_SAFE_STATIC_LOCAL(SparseHistogram,
                                   idb_request_loader_read_errors_histogram,
                                   ("Storage.Blob.IDBRequestLoader.ReadError"));
-  idb_request_loader_read_errors_histogram.Sample(loader_->GetNetError());
+  idb_request_loader_read_errors_histogram.Sample(
+      std::max(0, -loader_->GetNetError()));
 
   ReportError();
 }