WebView: Pass channel to variations

Since b66db7c4, WebView can now get the
channel. Variations uses this to filter experiments.

Stand-alone WebView will report "unknown" channel. Experiments which do
not specify any channel will apply to stand-alone WebView; see
CheckStudyChannel in study_filtering.cc.

BUG=758290

Change-Id: I6174b63c6995cb950e5ec349891ad579d24e7396
Reviewed-on: https://chromium-review.googlesource.com/1041206Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Paul Miller <paulmiller@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555802}

android_webview/browser/aw_variations_service_client.cc

@@ -7,6 +7,7 @@
 #include "base/bind.h"
 #include "base/threading/thread_restrictions.h"
 #include "build/build_config.h"
+#include "components/version_info/android/channel_getter.h"
 #include "components/version_info/version_info.h"
 
 namespace android_webview {
@@ -45,9 +46,7 @@ AwVariationsServiceClient::GetNetworkTimeTracker() {
 }
 
 version_info::Channel AwVariationsServiceClient::GetChannel() {
-  // TODO(kmilka): Investigate the proper value to return here so experiments
-  // are correctly filtered.
-  return version_info::Channel::UNKNOWN;
+  return version_info::GetChannel();
 }
 
 bool AwVariationsServiceClient::OverridesRestrictParameter(