[SafeBrowsing] Block dangerous unchecked downloads based on a Finch trial.

chrome/browser/safe_browsing/unverified_download_policy.cc

+// Copyright 2015 The Chromium Authors. All rights reserved.  Use of this source
+// code is governed by a BSD-style license that can be found in the LICENSE
+// file.
+
+#include "chrome/browser/safe_browsing/unverified_download_policy.h"
+
+#include "base/files/file_path.h"
+#include "base/metrics/histogram_macros.h"
+#include "chrome/browser/safe_browsing/unverified_download_field_trial.h"
+#include "chrome/common/safe_browsing/download_protection_util.h"
+
+namespace safe_browsing {
+
+bool IsUnverifiedDownloadAllowed(const base::FilePath& file) {
+  // Currently, the policy is determined entirely by a field trial.
+  bool is_allowed = IsUnverifiedDownloadAllowedByFieldTrial(file);
+  int uma_file_type =
+      download_protection_util::GetSBClientDownloadExtensionValueForUMA(file);
+  if (is_allowed) {
+    UMA_HISTOGRAM_ENUMERATION(
+        "SafeBrowsing.UnverifiedDownloads.Allowed", uma_file_type,
+        download_protection_util::kSBClientDownloadExtensionsMax);
+  } else {
+    UMA_HISTOGRAM_ENUMERATION(
+        "SafeBrowsing.UnverifiedDownloads.Blocked", uma_file_type,
+        download_protection_util::kSBClientDownloadExtensionsMax);

[Ilya Sherman, 2015/10/23 20:04:42]

Could you use UMA_HISTOGRAM_SPARSE_SLOWLY for these histograms, rather than a
dense histogram representation?  I'm guessing that most of the 100+ buckets are
not hit in any given run of Chrome.

(For bonus points, it would be awesome if you followed up with a CL that updates
any existing macro invocations, where safe to do so.)

[asanka, 2015/10/26 14:50:52]

Done. You're correct that these enumerations are not hit very frequently and it
makes sense to use a sparse histogram.

TBH, I didn't fully realize the difference between LinearHistogram and
SparseHistogram until just now when I looked through them. I'll upload a CL
shortly to switch over the user histograms that use the
SBClientDownloadExtensions.

+  }
+  return is_allowed;
+}
+
+}  // namespace safe_browsing