Revert of "Revert of Moved text decoding to the parser thread"

Reverts https://codereview.chromium.org/118573002/ and adds a crash fix.

Original commit message:
>>Moved text decoding to the parser thread.

>>When the HTTPDocumentParser defers the document parsing to the background parser thread, it'll now also pass ownership of its TextResourceDecoder so the parser thread can do the decoding as well.

R=abarth@chromium.org,eseidel@chromium.org
BUG=277886

Review URL: https://codereview.chromium.org/129123002

git-svn-id: svn://svn.chromium.org/blink/trunk@164797 bbb929c8-8fbe-4397-9dbb-9b2b20218538