Clean up usage of CSP functions

I was very confused by the ScriptLoader code which called
CSP::allowScriptNonce and the corresponding variable
which was called isValidScriptNonce.  I searched around
for nonce functions and found that they only checked
the nonce (not all policies) and I originally thought
this was an error!  However after further investigation
I found that allowScriptNonce actually checks all active
CSPs and returns true only if they all pass, thus making
the following line (which disables further CSP checks
on the load) OK.

I renamed these allowScriptNonce and allowStyleNonce
functions to have a 'With' in their name and added
a comment in the header explaining their behavior.

While making this change, I also reduced the verbosity
of several repeated uses of the string ContentSecurityPolicy.
I think most blink hackers would be able to look up that CSP
refers to ContentSecurityPolicy.

I removed several redundant calls to Document::contentSecurityPolicy
(again just caused the lines to be needlessly verbose).

Finally I made ScriptLoader::executeScript use the preferred
early-return pattern instead of a long indented block
for the !frame case.  I suspect that the !frame check
can actually be moved much earlier in the function
or even turned into an ASSERT.

This is still my long yack-shave to actually make the
preloader correct so I can fix the bugs which my
patch to make the HTML parser yield more agressively
(and thus not starve the preloader) possible to land:
https://codereview.chromium.org/258013009/

BUG=356292

Review URL: https://codereview.chromium.org/353873003

git-svn-id: svn://svn.chromium.org/blink/trunk@177065 bbb929c8-8fbe-4397-9dbb-9b2b20218538