Implement prefetching in chrome

With this CL (see also issue 2910009), chrome will support basic
prefetching.  You can optionally deactivate prefetching with the
command line argument --disable-prefetch.

A new RequestPriority was created as well, IDLE, which is lower
than LOWEST.  Unfortunately, SPDY has only two bits for priority, so
as a temporary measure (pending SPDY v3 which will have three), we
have a mapping in SPDY that folds net::LOWEST and net::IDLE together.

BUG=13505
TEST=http://gemal.dk/browserspy/prefetch.php

Review URL: http://codereview.chromium.org/3050016

git-svn-id: http://src.chromium.org/svn/trunk/src/build@54421 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

features_override.gypi

@@ -28,6 +28,7 @@
       'ENABLE_INPUT_SPEECH=1',
       'ENABLE_JAVASCRIPT_DEBUGGER=1',
       'ENABLE_JSC_MULTIPLE_THREADS=0',
+      'ENABLE_LINK_PREFETCH=0',
       'ENABLE_METER_TAG=1',
       'ENABLE_NOTIFICATIONS=1',
       'ENABLE_OFFLINE_WEB_APPLICATIONS=1',