Enable error page isolation.

Error page isolation was introduced in https://crrev.com/c/762520 but it
had some function bugs (e.g. https://crbug.com/840485). There have been
multiple fixes since then and it should be in a shape to try and enable
it again.

Bug: 838161
Change-Id: Ic3815e26eca9850c14e2cd925ed037223f4ded11
Reviewed-on: https://chromium-review.googlesource.com/1115203
Commit-Queue: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570951}

chrome/browser/chrome_content_browser_client.cc

@@ -1700,9 +1700,7 @@ bool ChromeContentBrowserClient::ShouldSwapBrowsingInstancesForNavigation(
 bool ChromeContentBrowserClient::ShouldIsolateErrorPage(bool in_main_frame) {
   // TODO(nasko): Consider supporting error page isolation in subframes if
   // Site Isolation is enabled.
-  // Note: It is temporarily disabled to investigate functional regression
-  // and process terminations. See https://crbug.com/838161.
-  return false;
+  return in_main_frame;
 }
 
 bool ChromeContentBrowserClient::ShouldAssignSiteForURL(const GURL& url) {

content/public/browser/content_browser_client.cc

@@ -173,7 +173,7 @@ bool ContentBrowserClient::ShouldSwapBrowsingInstancesForNavigation(
 }
 
 bool ContentBrowserClient::ShouldIsolateErrorPage(bool in_main_frame) {
-  return false;
+  return in_main_frame;
 }
 
 std::unique_ptr<media::AudioManager> ContentBrowserClient::CreateAudioManager(