add a flag to enable/disable download interception on android

chrome/browser/renderer_host/chrome_resource_dispatcher_host_delegate.cc

Index: chrome/browser/renderer_host/chrome_resource_dispatcher_host_delegate.cc
diff --git a/chrome/browser/renderer_host/chrome_resource_dispatcher_host_delegate.cc b/chrome/browser/renderer_host/chrome_resource_dispatcher_host_delegate.cc
index 0f8a31b584d86bd7990de5e7977ffe6e44236d9b..f45e77d5cc14a62e259b928a276de4dc5d80ba34 100644
--- a/chrome/browser/renderer_host/chrome_resource_dispatcher_host_delegate.cc
+++ b/chrome/browser/renderer_host/chrome_resource_dispatcher_host_delegate.cc
@@ -92,6 +92,8 @@
 #endif
 
 #if BUILDFLAG(ANDROID_JAVA_UI)
+#include "base/command_line.h"
+#include "base/metrics/field_trial.h"
 #include "chrome/browser/android/intercept_download_resource_throttle.h"
 #endif
 
@@ -268,6 +270,15 @@ void AppendComponentUpdaterThrottles(
 }
 #endif  // !defined(DISABLE_NACL)
 
+#if BUILDFLAG(ANDROID_JAVA_UI)
+const char kDownloadInterception[] = "EnableDownloadInterception";
+
+bool IsDownloadInterceptionEnabled() {
+  return base::StartsWith(
+      base::FieldTrialList::FindFullName(kDownloadInterception), "Enabled",
+      base::CompareCase::INSENSITIVE_ASCII);

[asanka, 2016/02/11 22:49:20]

Let's move these out to a separate file.

[qinmin, 2016/02/12 00:31:56]

Done. Moved to InterceptDownloadResourceThrottle class.

+}
+#endif  // BUILDFLAG(ANDROID_JAVA_UI)
 }  // namespace
 
 ChromeResourceDispatcherHostDelegate::ChromeResourceDispatcherHostDelegate()
@@ -418,9 +429,12 @@ void ChromeResourceDispatcherHostDelegate::DownloadStarting(
         download_request_limiter_, info->GetWebContentsGetterForRequest(),
         request->url(), request->method()));
 #if BUILDFLAG(ANDROID_JAVA_UI)
-    throttles->push_back(
-        new chrome::InterceptDownloadResourceThrottle(
-            request, child_id, route_id, request_id));
+    if (IsDownloadInterceptionEnabled() ||
+        base::CommandLine::ForCurrentProcess()->HasSwitch(
+            switches::kEnableDownloadInterception)) {

[asanka, 2016/02/11 22:49:20]

Why is disabling the default?

Also, if the feature was enabled by the field trial, the user can't override it.

[qinmin, 2016/02/12 00:31:56]

Done. Now the remote server will disable download interception, and locally we
can override that. The default behavior is to always intercept.

+      throttles->push_back(new chrome::InterceptDownloadResourceThrottle(
+          request, child_id, route_id, request_id));
+    }
 #endif
   }