Introduce DEBUG module which traces public API calls

This patch improves on DEBUG module to trace all puppeteer's
public API calls.

References #89.

lib/helper.js

@@ -91,6 +91,41 @@ class Helper {
       });
     }
   }
+
+  /**
+   * @param {!Object} classType
+   */
+  static tracePublicAPI(classType) {
+    let className = classType.prototype.constructor.name;
+    className = className.substring(0, 1).toLowerCase() + className.substring(1);
+    const debug = require('debug')(`puppeteer:${className}`);
+    if (!debug.enabled)
+      return;
+    for (let methodName of Reflect.ownKeys(classType.prototype)) {
+      const method = Reflect.get(classType.prototype, methodName);
+      if (methodName === 'constructor' || methodName.startsWith('_') || typeof method !== 'function')
+        continue;
+      Reflect.set(classType.prototype, methodName, function(...args) {
+        let argsText = args.map(stringifyArgument).join(', ');
+        let callsite = `${className}.${methodName}(${argsText})`;
+        debug(callsite);
+        return method.call(this, ...args);
+      });
+    }
+
+    /**
+     * @param {!Object} arg
+     * @return {string}
+     */
+    function stringifyArgument(arg) {
+      if (typeof arg !== 'function')
+        return JSON.stringify(arg);
+      let text = arg.toString().split('\n').map(line => line.trim()).join('');
+      if (text.length > 20)
+        text = text.substring(0, 20) + '…';
+      return `"${text}"`;
+    }
+  }
 }
 
 module.exports = Helper;