[Blink-scheduler] Correct description for [Background/Foreground]RendererMainThreadLoad histograms.

r413783 introduced renderer load level monitoring and introduced the
RendererScheduler.[Background/ForegroundRendererMainThreadLoad] histograms.
r423509 changed how often these histograms were recorded without updating the
histogram descriptions. This CL updates the descriptions to correctly reflect
that these are recorded at most once per minute per renderer.

BUG=None

Review-Url: https://codereview.chromium.org/2856273003
Cr-Commit-Position: refs/heads/master@{#469273}

tools/metrics/histograms/histograms.xml

@@ -58416,7 +58416,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
     time spent on running tasks.
 
     This metric is emitted when the renderer main thread task is completed or
-    renderer is backgrounded or foregrounded, at most once per second per
+    renderer is backgrounded or foregrounded, at most once per minute per
     renderer amortized.
   </summary>
 </histogram>
@@ -58437,7 +58437,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
     time spent on running tasks.
 
     This metric is emitted when the renderer main thread task is completed or
-    renderer is backgrounded or foregrounded, at most once per second per
+    renderer is backgrounded or foregrounded, at most once per minute per
     renderer amortized.
   </summary>
 </histogram>