DevTools: Update protocol description for CSS.stopRuleUsageTracking

Change-Id: I8e9916fdd83b22f998f666f34df56a3d2552094b
Reviewed-on: https://chromium-review.googlesource.com/891809
Commit-Queue: Paul Irish <paulirish@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532668}

third_party/WebKit/Source/core/inspector/browser_protocol.json

@@ -2234,7 +2234,7 @@
                 },
                 {
                     "name": "stopRuleUsageTracking",
-                    "description": "The list of rules with an indication of whether these were used",
+                    "description": "Stop tracking rule usage and return the list of rules that were used since last call to\n`takeCoverageDelta` (or since start of coverage instrumentation)",
                     "returns": [
                         {
                             "name": "ruleUsage",

third_party/WebKit/Source/core/inspector/browser_protocol.pdl

@@ -1019,7 +1019,8 @@ experimental domain CSS
   # Enables the selector recording.
   command startRuleUsageTracking
 
-  # The list of rules with an indication of whether these were used
+  # Stop tracking rule usage and return the list of rules that were used since last call to 
+  # `takeCoverageDelta` (or since start of coverage instrumentation)
   command stopRuleUsageTracking
     returns
       array of RuleUsage ruleUsage