Lower priority of async scripts

All scripts currently load at a medium priority and compete with each
other for resources. Async scripts do not block the parser and should be
loaded at a lower priority than those that do.

This patch lowers the priority of scripts with async or defer attributes
and scripts that were injected dynamically to load at a "low" priority
which is below blocking scripts but above (non-visible) images.

Visible images are loaded at the same priority but with a higher
intra-priority based on the screen area they cover so visible images
can preempt async scripts once layout is done (a good thing).

We want to keep scripts at a higher priority than (non-visible) images
for a couple of reasons:

1 - Scripts may cause other resource loads while images do not
(generally). There is potential to push out the onload time of the page
if a script with lots of follow-on content is loaded at the end after
images.

2 - By the time the code executes to inject the script the preload
parser may have already discovered the bulk of the images on the page.

Test results: https://docs.google.com/spreadsheet/ccc?key=0As3TLupYw2RedC1RUHlMRHhFY3RoVHEtc2FhbXQ1aWc&usp=sharing

Slight improvements (~1%) to onload, render and DOM Content Loaded.
1-2% improvement on Speed Index.

BUG=408229

Review URL: https://codereview.chromium.org/514893002

git-svn-id: svn://svn.chromium.org/blink/trunk@181842 bbb929c8-8fbe-4397-9dbb-9b2b20218538