chore(doclint): support classes inheritance (#935)

This patch:
- gives meaningful names to doclint tests
- supports classes inheritance in documentation linter. When class A
  extends class B, all methods of class B are added to documentation of
  class A.

This is a prerequisite for Object Handles: ElementHandle will be
extending ObjectHandle.

References #382

utils/doclint/check_public_api/test/md-builder-common/doc.md

@@ -0,0 +1,19 @@
+### class: Foo
+
+This is a class.
+
+#### event: 'frame'
+- <[Frame]> 
+
+This event is dispatched.
+
+#### foo.$(selector)
+- `selector` <[string]> A selector to query page for
+- returns: <[Promise]<[ElementHandle]>>
+
+The method runs document.querySelector.
+
+#### foo.url
+- <[string]>
+
+Contains the URL of the request.

utils/doclint/check_public_api/test/md-builder-common/result.txt

@@ -0,0 +1,32 @@
+{
+  "classes": [
+    {
+      "name": "Foo",
+      "members": [
+        {
+          "name": "frame",
+          "type": "event",
+          "hasReturn": false,
+          "async": false,
+          "args": []
+        },
+        {
+          "name": "$",
+          "type": "method",
+          "hasReturn": true,
+          "async": false,
+          "args": [
+            "selector"
+          ]
+        },
+        {
+          "name": "url",
+          "type": "property",
+          "hasReturn": false,
+          "async": false,
+          "args": []
+        }
+      ]
+    }
+  ]
+}