Revert of Make data: urls always parse async (https://codereview.chromium.org/210253003/)

Reason for revert:
Seems to have broken many browser_tests:
http://build.chromium.org/p/chromium.webkit/builders/Linux%20Tests%20%28dbg%29/builds/2364/steps/browser_tests/logs/stdio
http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_chromeos_rel/builds/18723/steps/browser_tests%20%28with%20patch%29

Original issue's description:
> Make data: urls always parse async
> 
> Turns out Chrome has long had the behavior of issuing
> loads and delivering load events async for data: urls,
> but not actually doing the parsing async. This changes
> makes the parsing of data:text/html content always async.
> 
> NOTE: The new async test (loader/iframe-sync-loads.html)
> does not change before before/after this test, it's
> there to confirm that we were already behaving async
> with regards to when the content appeared in the iframe.
> 
> This finally makes it possible to take a JS string
> and run it through the HTML parser async.  I'll fix
> <iframe srcdoc> to also be async in a follow-up patch.
> 
> I wrote a test to check sync/async behavior of various
> loading methods for iframes. Gecko has async behavior
> for all 3 of these cases in my test, where as Chrome
> on Linux has sync behavior for javascript:
> 
> This is covered by 3 additional tests:
> fast/dom/document-contentType-data-uri.html:
> - text/html content is now async, matches FF
> 
> accessibility/loading-iframe-sends-notification.html
> - had to re-write this test to not assume that the iframe
> would be laid out before its onload was fired.
> 
> scrollingcoordinator/non-fast-scrollable-region-transformed-iframe.html
> - This was hitting crbug.com/365509 which I worked around by
> calling setUseMockScrollbars earlier.
> 
> BUG=308321, 365509
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=172401

TBR=abarth@chromium.org,japhet@chromium.org,dmazzoni@chromium.org,eseidel@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=308321, 365509

Review URL: https://codereview.chromium.org/259443002

git-svn-id: svn://svn.chromium.org/blink/trunk@172426 bbb929c8-8fbe-4397-9dbb-9b2b20218538