DevTools: fix browser-side navigation of paused page inside renderer

When navigating page inside the same renderer with browser-side
navigation, we are too late with unpausing the Page.

As a result, the page still considers itself paused and hits
a DCHECK in DocumentLoader::FinishedLoading.

R=dgozman

Change-Id: I0998b94c4a379c23bd44d56a6cb7c9fb4aa3bb5c
Reviewed-on: https://chromium-review.googlesource.com/1132378
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574016}

third_party/WebKit/LayoutTests/inspector-protocol/debugger/debugger-navigate-while-paused-expected.txt

+Tests that we can navigate away from paused page.
+SUCCESS: Paused
+SUCCESS: navigated from paused page.
+

third_party/WebKit/LayoutTests/inspector-protocol/debugger/debugger-navigate-while-paused.js

+(async function(testRunner) {
+  const {page, session, dp} = await testRunner.startBlank('Tests that we can navigate away from paused page.');
+
+  await dp.Runtime.enable();
+  await dp.Debugger.enable();
+  await dp.Page.enable();
+  await dp.Page.setLifecycleEventsEnabled({enabled: true});
+
+  // Start tight loop in page.
+  session.evaluate(`debugger;`),
+  await dp.Debugger.oncePaused(),
+  testRunner.log('SUCCESS: Paused');
+
+  await dp.Page.navigate({url: 'about:blank'});
+  await dp.Page.onceLifecycleEvent(event => event.params.name === 'load');
+
+  testRunner.log('SUCCESS: navigated from paused page.');
+  testRunner.completeTest();
+})

third_party/blink/renderer/core/exported/web_dev_tools_agent_impl.cc

@@ -168,18 +168,17 @@ class ClientMessageLoopAdapter : public MainThreadDebugger::ClientMessageLoop {
 
     // 3. Process messages until quitNow is called.
     message_loop_->Run();
-
-    // 4. Resume active objects
-    WebView::DidExitModalLoop();
-
-    // 5. Enable input events.
-    WebFrameWidgetBase::SetIgnoreInputEvents(false);
   }
 
   void QuitNow() override {
     if (running_for_debug_break_) {
       running_for_debug_break_ = false;
+      // Undo steps (3), (2) and (1) from above.
+      // NOTE: This code used to be above right after the |mesasge_loop_->Run()|
+      // code, but it is moved here to support browser-side navigation.
       message_loop_->QuitNow();
+      WebView::DidExitModalLoop();
+      WebFrameWidgetBase::SetIgnoreInputEvents(false);
     }
   }