Force synchronized parser for substitute main resource load marked forceSynchronousLoad.

Before this CL, synchronized parsing for forceSynchronousLoad
substituteData was not enforced. The flag was simply ignored in parsing
code path.

This bug has not surfaced because all substituteData which required
synchronized loading were used against about:blank URL, and
HTMLDocumentParserOptions had a special casing that about:blank urls
required synchronous parsing.

This CL explicitly enforces synchronous parse for forceSynchronousLoad
substituteData, so that flag is actually used.
This is also a step toward removing special casing against about:black
URL in HTMLDocumentParser.

BUG=421289

Review URL: https://codereview.chromium.org/646913002

git-svn-id: svn://svn.chromium.org/blink/trunk@183570 bbb929c8-8fbe-4397-9dbb-9b2b20218538