Update UKM API docs with current best practices.

Change-Id: Ida79579f16af6c2d0dcd97a7d972b2444f2280c3
Reviewed-on: https://chromium-review.googlesource.com/827541Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524446}

services/metrics/public/cpp/mojo_ukm_recorder.h

@@ -24,13 +24,11 @@ namespace ukm {
  *
  * Usage Example:
  *
- *  ukm::mojom::UkmRecorderInterfacePtr interface;
- *  content::RenderThread::Get()->GetConnector()->BindInterface(
- *      content::mojom::kBrowserServiceName, mojo::MakeRequest(&interface));
- *  ukm::MojoUkmRecorder ukm_recorder(std::move(interface));
+ *  std::unique_ptr<ukm::MojoUkmRecorder> ukm_recorder =
+ *      ukm::MojoUkmRecorder::Create(context()->connector());
  *  ukm::builders::MyEvent(source_id)
  *      .SetMyMetric(metric_value)
- *      .Record(ukm_recorder);
+ *      .Record(ukm_recorder.get());
  */
 class METRICS_EXPORT MojoUkmRecorder : public UkmRecorder {
  public:

services/metrics/ukm_api.md

@@ -39,24 +39,30 @@ In order to record UKM events, your code needs a UkmRecorder object, defined by
 
 There are two main ways of getting a UkmRecorder instance.
 
-1) Use ukm::UkmRecorder::Get().  This only works from the UI thread of the Browser process.
+1) Use ukm::UkmRecorder::Get().  This currently only works from the Browser process.
 
 2) Use a service connector and get a MojoUkmRecorder.
 
 ```
-ukm::mojom::UkmRecorderInterfacePtr interface;
-service_connector->BindInterface(
-      content::mojom::kBrowserServiceName,
-      mojo::MakeRequest(&interface));
-ukm::MojoUkmRecorder ukm_recorder(std::move(interface));
+std::unique_ptr<ukm::MojoUkmRecorder> ukm_recorder =
+    ukm::MojoUkmRecorder::Create(context()->connector());
+ukm::builders::MyEvent(source_id)
+    .SetMyMetric(metric_value)
+    .Record(ukm_recorder.get());
 ```
 
-Note: This will eventually move out of the browser service.
-
 ## Get a ukm::SourceId
 
 UKM identifies navigations by thier source ID and you'll need to associate and ID with your event in order to tie it to a main frame URL.  Preferrably, get an existing ID for the navigation from another object.
 
+The main methods for doing this are using one of the following methods:
+
+```
+ukm::SourceId source_id = GetSourceIdForWebContentsDocument(web_contents);
+ukm::SourceId source_id = ukm::ConvertToSourceId(
+    navigation_handle->GetNavigationId(), ukm::SourceIdType::NAVIGATION_ID);
+```
+
 Currently, however, the code for passing these IDs around is incomplete so you may need to temporarily create your own IDs and associate the URL with them.  Example:
 
 ```