Always preload all tokens before parsing

If Android WebView tests break from this change,
please disable those tests: crbug.com/387101

This change is in preparation for re-writing the path
through which tokens are handled on the main thread
to allow them to be batched and deferred until more
important work is done:
https://codereview.chromium.org/258013009/

Currently the threaded parser uses the main thread's
MessageQueue as its work-queue and we'll commonly
queue 10 chunks of work in the main thread MessageQueue
even if the first chunk takes a long time we'll still have to
process the next 9 before we can do more important things
like put up a frame because we don't have a separate
queue for parser work.  258013009 will fix that.

http/tests/security/script-crossorigin-loads-correctly-credentials-2.html
originally started failing with this change and it
turns out that always-preloading found a real bug
in CORS handling with preloads. The ResourceFetcher
was incorrectly always reusing an in-flight preload
request, even if the CORS state didn't match. Nate
helped me re-order the calls in requestResource to fix it.

This change also discovered 2 other bugs in the preloader.
1. a preload request != LinkPrefect request
and one will cancel the other. crbug.com/379893
2. The preloader has no concept of script type and will
blindly issue preloads for all <script href> regardless
of <script type>.  I decided this didn't matter in practice.
Making the <script type> handling threadsafe isn't
worth the trouble.

I would anticipate that this change might make some
speed-index scores better (we'll now be preloading
all tokens slightly sooner, especially those
which are in the same chunk as a long inline
script block for instance), but may make non-network
PLT scores lower (due to extra mallocs required
to process every chunk since more loads will
be preloaded just before their actually loaded).

BUG=356292

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175427

Review URL: https://codereview.chromium.org/302063002

git-svn-id: svn://svn.chromium.org/blink/trunk@176642 bbb929c8-8fbe-4397-9dbb-9b2b20218538