Set page_url in CSBRR proto sent on APK downloads

Also, add a comment in the proto.
See http://b/123719599 for details.

R=drubery

Bug: 907280,927831
Change-Id: Idc690f25bfc3b909eead54c0184184bcf0df8a32
Reviewed-on: https://chromium-review.googlesource.com/c/1450415Reviewed-by: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628425}

chrome/browser/safe_browsing/telemetry/android/android_telemetry_service.cc

@@ -259,6 +259,7 @@ void AndroidTelemetryService::MaybeSendApkDownloadReport(
   report->set_type(
       safe_browsing::ClientSafeBrowsingReportRequest::APK_DOWNLOAD);
   report->set_url(item->GetOriginalUrl().spec());
+  report->set_page_url(item->GetTabUrl().spec());
 
   // Fill referrer chain.
   content::WebContents* web_contents =

components/safe_browsing/proto/csd.proto

@@ -1180,7 +1180,12 @@ message ClientSafeBrowsingReportRequest {
 
   // URL of the page in the address bar.
   optional string url = 1;
+
+  // Must be set if the ReportType is not one of DANGEROUS_DOWNLOAD_RECOVERY,
+  // DANGEROUS_DOWNLOAD_WARNING, DANGEROUS_DOWNLOAD_BY_API,
+  // DANGEROUS_DOWNLOAD_OPENED, URL_PASSWORD_PROTECTION_PHISHING.
   optional string page_url = 2;
+
   optional string referrer_url = 3;
 
   repeated Resource resources = 4;