Fix PPAPIDownloadRequest.RequestDuration metric

PPAPIDownloadRequest.RequestDuration is always zero. This is because
UMA_HISTOGRAM_TIMES macro cannot handle negative number. This Cl fixes
this issue.

Bug: 633341
Change-Id: I8dd1200ed7f28e746469b4d82ef0e927a5080a88
Reviewed-on: https://chromium-review.googlesource.com/777887
Commit-Queue: Jialiu Lin <jialiul@chromium.org>
Commit-Queue: Nathan Parker <nparker@chromium.org>
Reviewed-by: Nathan Parker <nparker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517657}

chrome/browser/safe_browsing/download_protection/ppapi_download_request.cc

@@ -287,7 +287,7 @@ void PPAPIDownloadRequest::Finish(RequestOutcome reason,
   UMA_HISTOGRAM_SPARSE_SLOWLY("SBClientDownload.PPAPIDownloadRequest.Result",
                               static_cast<int>(response));
   UMA_HISTOGRAM_TIMES("SBClientDownload.PPAPIDownloadRequest.RequestDuration",
-                      start_time_ - base::TimeTicks::Now());
+                      base::TimeTicks::Now() - start_time_);
   if (!callback_.is_null())
     base::ResetAndReturn(&callback_).Run(response);
   fetcher_.reset();