DevTools: add support for polling for coverage data in CSS agent

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

Index: third_party/WebKit/Source/core/inspector/browser_protocol.json
diff --git a/third_party/WebKit/Source/core/inspector/browser_protocol.json b/third_party/WebKit/Source/core/inspector/browser_protocol.json
index cab338945b8c153c303e4a52b6f618b4b7c645ca..196363cbe9c3e8428981df9100819956de4e9d8d 100644
--- a/third_party/WebKit/Source/core/inspector/browser_protocol.json
+++ b/third_party/WebKit/Source/core/inspector/browser_protocol.json
@@ -2823,7 +2823,7 @@
                     { "name": "endOffset", "type": "number", "description": "Offset of the end of the rule body from the beginning of the stylesheet." },
                     { "name": "used", "type": "boolean", "description": "Indicates whether the rule was actually used by some element in the page." }
                 ],
-                "description": "CSS rule usage information.",
+                "description": "CSS coverage information.",
                 "experimental": true
             },
             {
@@ -3195,13 +3195,21 @@
                 ],
                 "description": "For the main document and any content documents, return the LayoutTreeNodes and a whitelisted subset of the computed style. It only returns pushed nodes, on way to pull all nodes is to call DOM.getDocument with a depth of -1.",
                 "experimental": true
-	    },
+            },
             {
                 "name": "startRuleUsageTracking",
                 "description": "Enables the selector recording.",
                 "experimental": true
             },
             {
+                "name": "takeCoverageDelta",
+                "description": "Obtain list of rules that became used since last call to this method (or since start of coverage instrumentation)",
+                "returns": [
+                    { "name": "coverage", "type": "array", "items": { "$ref": "RuleUsage" } }
+                ],
+                "experimental": true
+            },
+            {
                 "name": "stopRuleUsageTracking",
                 "returns": [
                     { "name": "ruleUsage", "type": "array", "items": { "$ref": "RuleUsage" } }