Revert of Revert "Moved text decoding to the parser thread" (https://codereview.chromium.org/133273007/)

Reason for revert:
High confidence of having identified the cause of the crash now, will apply that in second changelist.

Original issue's description:
> Revert "Moved text decoding to the parser thread"
> 
> This reverts 164797
> 
> R=abarth@chromium.org,eseidel@chromium.org
> BUG=329603
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=165102

TBR=abarth@chromium.org,eseidel@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=329603

Review URL: https://codereview.chromium.org/141143007

git-svn-id: svn://svn.chromium.org/blink/trunk@165488 bbb929c8-8fbe-4397-9dbb-9b2b20218538