Update DeepScanAccessPoint comment to clarify it's usage

Now that the access point is used for something more than just metrics,
it should be clear that the existing values should not be used again
for other purposes.

Change-Id: I92eeba7e9c2ea2605b3d58476a414355f53de6da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2028167Reviewed-by: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Dominique Fauteux-Chapleau <domfc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736845}

chrome/browser/safe_browsing/cloud_content_scanning/deep_scanning_utils.h

@@ -27,15 +27,26 @@ void MaybeReportDeepScanningVerdict(Profile* profile,
                                     BinaryUploadService::Result result,
                                     DeepScanningClientResponse response);
 
-// Access points used to record UMA metrics. Adding an access point here
-// requires updating histograms.xml by adding histograms with names
+// Access points used to record UMA metrics and specify which code location is
+// initiating a deep scan. Any new caller of
+// DeepScanningDialogDelegate::ShowForWebContents should add an access point
+// here instead of re-using an existing value. histograms.xml should also be
+// updated by adding histograms with names
 //   "SafeBrowsing.DeepScan.<access-point>.BytesPerSeconds"
 //   "SafeBrowsing.DeepScan.<access-point>.Duration"
 //   "SafeBrowsing.DeepScan.<access-point>.<result>.Duration"
+// for the new access point and every possible result.
 enum class DeepScanAccessPoint {
+  // A deep scan was initiated from downloading 1+ file(s).
   DOWNLOAD,
+
+  // A deep scan was initiated from uploading 1+ file(s) via a system dialog.
   UPLOAD,
+
+  // A deep scan was initiated from drag-and-dropping text or 1+ file(s).
   DRAG_AND_DROP,
+
+  // A deep scan was initiated from pasting text.
   PASTE,
 };
 std::string DeepScanAccessPointToString(DeepScanAccessPoint access_point);