[DevTools] Add CDP test for Page domain

- Test that both DomContentLoaded and Load events fire correctly on page navigations

Change-Id: I4c2e04b49ca76c31e1202584eaa63aa1c57c2696
Reviewed-on: https://chromium-review.googlesource.com/c/1479639Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: James Lissiak <jalissia@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#634412}

third_party/blink/web_tests/inspector-protocol/page/pageDomContentBeforeLoadEventFired-expected.txt

+Tests Page.domContentEventFired is triggered before Page.loadEventFired for a non-blank page
+{
+    method : Page.domContentEventFired
+    params : {
+        timestamp : <number>
+    }
+    sessionId : <string>
+}
+{
+    method : Page.loadEventFired
+    params : {
+        timestamp : <number>
+    }
+    sessionId : <string>
+}
+

third_party/blink/web_tests/inspector-protocol/page/pageDomContentBeforeLoadEventFired.js

+(async function (testRunner) {
+  const { page, session, dp } = await testRunner.startBlank(
+    'Tests Page.domContentEventFired is triggered before Page.loadEventFired for a non-blank page');
+
+  await dp.Page.enable();
+
+  const htmlPage = testRunner.url('../resources/dom-snapshot.html');
+  dp.Page.navigate({ url: htmlPage });
+  const domContentEvent = await dp.Page.onceDomContentEventFired();
+  testRunner.log(domContentEvent);
+  const loadEvent = await dp.Page.onceLoadEventFired();
+  testRunner.log(loadEvent);
+
+  testRunner.completeTest();
+})

third_party/blink/web_tests/inspector-protocol/page/pageLoadBeforeDomContentEventFired-expected.txt

+Tests Page.loadEventFired is triggered before Page.domContentEventFired for a blank page
+{
+    method : Page.loadEventFired
+    params : {
+        timestamp : <number>
+    }
+    sessionId : <string>
+}
+{
+    method : Page.domContentEventFired
+    params : {
+        timestamp : <number>
+    }
+    sessionId : <string>
+}
+

third_party/blink/web_tests/inspector-protocol/page/pageLoadBeforeDomContentEventFired.js

+(async function (testRunner) {
+  const { page, session, dp } = await testRunner.startBlank(
+    'Tests Page.loadEventFired is triggered before Page.domContentEventFired for a blank page');
+
+  await dp.Page.enable();
+
+  const htmlPage = testRunner.url('../resources/blank.html');
+  dp.Page.navigate({ url: htmlPage });
+  const loadEvent = await dp.Page.onceLoadEventFired();
+  testRunner.log(loadEvent);
+  const domContentEvent = await dp.Page.onceDomContentEventFired();
+  testRunner.log(domContentEvent);
+
+  testRunner.completeTest();
+})