Oilpan: Prepare to move AbstractSQLStatement, SQLStatement, and SQLCallbackWrapper<T> to Oilpan heap.

SQLCallbackWrapper has code to call ExecutionContext::deref() in the origin
thread. This is unnecessary in Oilpan because implementation classes of
ExecutionContext are already on-heap and they are always destructed in their
origin threads.

This is a prepration to make ExecutionContext GarbageCollected.

BUG=357163

Review URL: https://codereview.chromium.org/311273005

git-svn-id: svn://svn.chromium.org/blink/trunk@175610 bbb929c8-8fbe-4397-9dbb-9b2b20218538