AX ITM: Updating m_pendingLoadingProgress can cause deadlock on AXIsolatedTree::m_cha...

AX ITM: Updating m_pendingLoadingProgress can cause deadlock on AXIsolatedTree::m_changeLogLock
https://bugs.webkit.org/show_bug.cgi?id=237402

Reviewed by Chris Fleizach.

AXIsolatedTree::m_pendingLoadingProgress is currently guarded by
AXIsolatedTree::m_changeLogLock. Because loading can happen at any time,
deadlocks can happen in this sequence:

  1. AXIsolatedTree::updateLoadingProgress is called on the main thread while
     the secondary thread holds the lock
  2. The secondary thread is holding the lock to service an AX request, and said
     AX request does something to call into the main thread (e.g. AXLOGs an isolated object,
     which causes a dispatch to the main thread as part of AXIsolatedObject::outerHTML).
  3. Deadlock

This patch fixes this by making m_loadingProgress threadsafe
via std::atomic<double> and removing m_pendingLoadingProgress.

This patch also removes an unnecessary acquisition of m_changeLogLock
in AXIsolatedTree::focusedNode(). This function is only called on the
secondary-thread, and only accesses secondary-thread safe functions
and member variables (nodeForID, m_focusedNodeID), so we don't need the lock.

* accessibility/isolatedtree/AXIsolatedTree.cpp:
(WebCore::AXIsolatedTree::focusedNode):
Removed unnecessary m_changeLogLock acquisition.
(WebCore::AXIsolatedTree::updateLoadingProgress):
Update m_loadingProgress directly instead of the now deleted intermediary
m_pendingLoadingProgress.
(WebCore::AXIsolatedTree::applyPendingChanges):
* accessibility/isolatedtree/AXIsolatedTree.h:
Remove m_pendingLoadingProgress.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@290833 268f45cc-cd09-0410-ab3c-d52691b4dbfc