Web SQL: Make sure DatabaseManager is used only in the main thread.

Only DatabaseManager::closeDatabasesImmediately could be called in non-main
threads. However, it just calls
DatabaseTracker::closeDatabasesImmediately(). This CL removes
DatabaseManager::closeDatabasesImmediately, and only the main thread uses
DatabaseManager.

So, we can remove DatabaseManager::m_contextMapLock.  We still need m_contextMap
because the main thread can have multiple Documents.

Oilpan: We can use PersistentHeapHashMap, and remove some |#if ENABLE(OILPAN)|.

BUG=347902,397429
TEST=No new tests because of no behavior changes.

Review URL: https://codereview.chromium.org/567453002

git-svn-id: svn://svn.chromium.org/blink/trunk@181810 bbb929c8-8fbe-4397-9dbb-9b2b20218538