UseCounter for <script src> tags with invalid type/lang attributes

This CL adds a UseCounter for <script> tags with invalid type/lang attributes
with src URLs.

Some old prefetch libraries may rely on a spec incompliant behavior of
HTMLPreloadScanner prefetching <script src> resources w/o checking the validity
of type/lang attributes.

This UseCounter is an attempt to measure how much <script> tags are there
which may trigger preload.

Note that this UseCounter overcount and doesn't check if the preload is
triggered explicitly by user javascript. For example, contentjs doesn't rely
on HTMLPreloadScanner to preload <script src>, but triggers resource fetch
explicitly by instantiating <object> tags.

BUG=329531

Review URL: https://codereview.chromium.org/678333002

git-svn-id: svn://svn.chromium.org/blink/trunk@184572 bbb929c8-8fbe-4397-9dbb-9b2b20218538