Refactoring: Expand comments in RFHM::CanUseSourceSiteInstance.

This CL breaks down a single boolean expression into separate,
explicit if statements.  This helps to improve the readability
of the function and add more comments that target individual
if statements (i.e. parts of the old boolean expression).

This CL has no intended behavior changes.

Bug: 1107269
Change-Id: Id309497e480d2ff278f0e381902b2626963094a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2336504
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795205}

content/browser/frame_host/render_frame_host_manager.cc

@@ -1892,11 +1892,19 @@ bool RenderFrameHostManager::CanUseSourceSiteInstance(
     SiteInstance* source_instance,
     bool was_server_redirect,
     bool is_failure) {
+  if (!source_instance)
+    return false;
+
   // We use the source SiteInstance in case of data URLs, about:srcdoc pages and
   // about:blank pages because the content is then controlled and/or scriptable
-  // by the source SiteInstance.
+  // by the initiator and therefore needs to stay in the |source_instance|.
+  if (!IsDataOrAbout(dest_url))
+    return false;
+
+  // One exception (where data URLs, about:srcdoc or about:blank pages are *not*
+  // controlled by the initiator) is when these URLs are reached via a server
+  // redirect.
   //
-  // One exception to this is when these URLs are reached via a server redirect.
   // Normally, redirects to data: or about: URLs are disallowed as
   // net::ERR_UNSAFE_REDIRECT, but extensions can still redirect arbitrary
   // requests to those URLs using webRequest or declarativeWebRequest API (for
@@ -1905,10 +1913,18 @@ bool RenderFrameHostManager::CanUseSourceSiteInstance(
   // redirects, the content is controlled by the extension (rather than by the
   // |source_instance|), so we don't use the |source_instance| for data: URLs if
   // there was a server redirect.
-  return source_instance && IsDataOrAbout(dest_url) &&
-         (!was_server_redirect || !dest_url.SchemeIs(url::kDataScheme)) &&
-         IsSiteInstanceCompatibleWithErrorIsolation(
-             source_instance, frame_tree_node_->IsMainFrame(), is_failure);
+  if (was_server_redirect && dest_url.SchemeIs(url::kDataScheme))
+    return false;
+
+  // Make sure that error isolation is taken into account.  See also
+  // ChromeNavigationBrowserTest.RedirectErrorPageReloadToAboutBlank.
+  if (!IsSiteInstanceCompatibleWithErrorIsolation(
+          source_instance, frame_tree_node_->IsMainFrame(), is_failure)) {
+    return false;
+  }
+
+  // Okay to use |source_instance|.
+  return true;
 }
 
 bool RenderFrameHostManager::IsCurrentlySameSite(RenderFrameHostImpl* candidate,