chrome: Prepare browser tests for PostTask refactor.

Updates prerender and safe browsing browser tests to initialize the
SafeBrowsingDatabaseManager after BrowserThreads were created (but
before they are started). This is required for the upcoming move of
BrowserThread::PostTask to base/task/post_task.h, as construction of
SafeBrowsingDatabaseManager obtains a TaskRunner for the IO thread.
In the future, this has to happen after thread initialization.

Bug: 878356
Change-Id: I1d4c614690ba3aacf4887fefe57d378c23668706
Reviewed-on: https://chromium-review.googlesource.com/1211735Reviewed-by: Jialiu Lin <jialiul@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589488}

chrome/browser/prerender/prerender_test_utils.cc

@@ -629,7 +629,8 @@ PrerenderInProcessBrowserTest::GetFakeSafeBrowsingDatabaseManager() {
           .get());
 }
 
-void PrerenderInProcessBrowserTest::SetUpInProcessBrowserTestFixture() {
+void PrerenderInProcessBrowserTest::CreatedBrowserMainParts(
+    content::BrowserMainParts* browser_main_parts) {
   safe_browsing_factory_->SetTestDatabaseManager(
       new test_utils::FakeSafeBrowsingDatabaseManager());
   safe_browsing::SafeBrowsingService::RegisterFactory(

chrome/browser/prerender/prerender_test_utils.h

@@ -311,7 +311,8 @@ class PrerenderInProcessBrowserTest : virtual public InProcessBrowserTest {
   ~PrerenderInProcessBrowserTest() override;
 
   void SetUpCommandLine(base::CommandLine* command_line) override;
-  void SetUpInProcessBrowserTestFixture() override;
+  void CreatedBrowserMainParts(
+      content::BrowserMainParts* browser_main_parts) override;
   void TearDownInProcessBrowserTestFixture() override;
   void SetUpOnMainThread() override;
   content::SessionStorageNamespace* GetSessionStorageNamespace() const;

chrome/browser/safe_browsing/safe_browsing_blocking_page_test.cc

@@ -366,15 +366,15 @@ class SafeBrowsingBlockingPageBrowserTest
 
   ~SafeBrowsingBlockingPageBrowserTest() override {}
 
-  void SetUp() override {
+  void CreatedBrowserMainParts(
+      content::BrowserMainParts* browser_main_parts) override {
     // Test UI manager and test database manager should be set before
-    // InProcessBrowserTest::SetUp().
+    // the browser is started but after threads are created.
     factory_.SetTestUIManager(new FakeSafeBrowsingUIManager());
     factory_.SetTestDatabaseManager(new FakeSafeBrowsingDatabaseManager());
     SafeBrowsingService::RegisterFactory(&factory_);
     SafeBrowsingBlockingPage::RegisterFactory(&blocking_page_factory_);
     ThreatDetails::RegisterFactory(&details_factory_);
-    InProcessBrowserTest::SetUp();
   }
 
   void TearDown() override {