DevTools: return the correct defaultExecutionContext

* Uses the sorted list by frame to determine the defaultExecutionContext.
* Fixes issue where Audits panel would audit the wrong URL.

BUG=742044

Change-Id: I56a1ebd0a0d1f2f9decd603f78c5b73921ea635e
Reviewed-on: https://chromium-review.googlesource.com/570921Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Patrick Hulce <phulce@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486888}

third_party/WebKit/Source/devtools/front_end/audits2/Audits2Panel.js

@@ -248,6 +248,8 @@ Audits2.Audits2Panel = class extends UI.PanelWithSidebar {
     if (!executionContext)
       return Promise.resolve();
 
+    // Evaluate location.href for a more specific URL than inspectedURL provides so that SPA hash navigation routes
+    // will be respected and audited.
     return new Promise(resolve => {
       executionContext.evaluate('window.location.href', 'audits', false, false, true, false, false, (object, err) => {
         if (!err && object) {

third_party/WebKit/Source/devtools/front_end/sdk/RuntimeModel.js

@@ -113,7 +113,7 @@ SDK.RuntimeModel = class extends SDK.SDKModel {
    * @return {?SDK.ExecutionContext}
    */
   defaultExecutionContext() {
-    for (var context of this._executionContextById.values()) {
+    for (var context of this.executionContexts()) {
       if (context.isDefault)
         return context;
     }