AW UMA: fix doc for kMetricsRecordingOnly switch

No change to logic.

The kDisableMetrics switch doesn't exist. I think the intended switch
name was kForceEnableMetricsReporting.

Bug: 994418
Test: N/A
Change-Id: I89e36fcf5f42225953c40c25ea00addcf1993185
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1809980
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697632}

components/metrics/metrics_switches.cc

@@ -8,10 +8,10 @@ namespace metrics {
 namespace switches {
 
 // Enables the recording of metrics reports but disables reporting. In contrast
-// to kDisableMetrics, this executes all the code that a normal client would
-// use for reporting, except the report is dropped rather than sent to the
-// server. This is useful for finding issues in the metrics code during UI and
-// performance tests.
+// to kForceEnableMetricsReporting, this executes all the code that a normal
+// client would use for reporting, except the report is dropped rather than sent
+// to the server. This is useful for finding issues in the metrics code during
+// UI and performance tests.
 const char kMetricsRecordingOnly[] = "metrics-recording-only";
 
 // Override the standard time interval between each metrics report upload for