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This is a partial reland of https://crrev.com/c/chromium/src/+/2062692.
We confirmed that the CL affected startup metrics on Mac. However, our
assessment is that the change is correct (restored tabs are painted
before non-restored tabs more often). As a precaution, the
original CL is split in two:
- Start loading restored tabs earlier (this CL)
- Enforce stricter requirements in FirstWebContentsProfiler (http://crrev.com/c/chromium/src/+/2097368)

Original change description:

On Windows, a visible restored tab starts loading in the following stack,
before FirstWebContentsProfiler is created:

  content::NavigationControllerImpl::LoadIfNecessary
  content::NavigationControllerImpl::SetActive
  ...
  content::WebContentsImpl::UpdateWebContentsVisibility
  content::WebContentsViewAura::UpdateWebContentsVisibility
  ...
  BrowserView::Show
  SessionRestoreImpl::ShowBrowser
  SessionRestoreImpl::RestoreTab
  SessionRestoreImpl::RestoreTabsToBrowser
  SessionRestoreImpl::ProcessSessionWindows
  SessionRestoreImpl::ProcessSessionWindowsAndNotify
  SessionRestoreImpl::Restore
  SessionRestore::RestoreSession
  ...
  StartupBrowserCreator::Start
  ...
  content::BrowserMainLoop::PreMainMessageLoopRun

On Mac, it starts loading in the following stack, after
FirstWebContentsProfiler is created:

  content::NavigationControllerImpl::LoadIfNecessary
  content::NavigationControllerImpl::SetActive
  ...
  content::WebContentsImpl::UpdateWebContentsVisibility
  content::WebContentsViewMac::OnWindowVisibilityChanged
  -[WebContentsViewCocoa updateWebContentsVisibility]
  -[WebContentsViewCocoa windowChangedOcclusionState:]
  ...
  base::sequence_manager::internal::ThreadControllerWithMessagePumpImpl::Run
  ...
  ChromeBrowserMainParts::MainMessageLoopRun

This prevents FirstWebContentsProfiler from having strict cross-platform
expectations about events it observes. With this CL, we ensure that visible
restored tab start loading in the following stack on all platforms:

  content::NavigationControllerImpl::LoadIfNecessary
  chrome::`anonymous namespace'::CreateRestoredTab
  chrome::AddRestoredTab
  ...
  SessionRestoreImpl::Restore
  SessionRestore::RestoreSession
  ...
  StartupBrowserCreator::Start
  ...
  content::BrowserMainLoop::PreMainMessageLoopRun

This allows us to enforce strict cross-platform expectations about events
observed by FirstWebContentsProfiler, and also have the nice benefits that
navigation isn't delayed until we get a window visibility update during a
Mac session restore.

Bug: 1035419, 1020549, 1022492
Change-Id: I2964de553c0195ba94a35dc41d17c4a28cc5d05d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095970
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749327}