Oilpan: Prepare to move DocumentParser and related classes to Oilpan heap.

This CL makes the following classes garbage-collectable with Oilpan transition
types:

- DocumentParser
-- RawDataDocumentParser
--- ImageDocumentParser
--- MediaDocumentParser
--- PluginDocumentParser
--- SinkDocumentParser
-- DecodedDataDocumentParser
--- ScriptableDocumentParser
---- HTMLDocumentParser
----- TextDocumentParser
---- XMLDocumentParser
--- HTMLViewSourceParser
- HTMLConstructionSite
- HTMLConstructionSiteTask
- HTMLTreeBuilder
- XMLError

When a Document is dying, we don't need to call DocumentParser::detach
in Oilpan because we don't need to clear RefPtr<>s used in a parser.
So, we should skip assertions in destructors of DocumentParser classes.
We need to stop background HTML parser in ~HTMLDocumentParser
because HTMLDocumentParser::detach might not be called.


BUG=357163
NOTRY=true

Review URL: https://codereview.chromium.org/290333009

git-svn-id: svn://svn.chromium.org/blink/trunk@174697 bbb929c8-8fbe-4397-9dbb-9b2b20218538