Move ContentLoader and FileLoader to this new way
as they involves IO. Will work on CacheLoader later.
Change StreamLoader to contain a Handler instead of
derive from a Handler so that the Handler can be
created in the thread where load() is called.
Rename StreamLoader's old "LoadListener mHandler"
to mLoadListener.
Remove unused import and unreachable exception.
Fix http://b/issue?id=2158613
This improved page_cycler performance in moz/intl by
10-30% as we are not blocked by IO any more.
ac75f56600