Style fixes for ContentSecurityPolicy.cpp.

Style issues cropped up when I moved CSP from core/page to core/frame[1].
This CL fixes them (mostly 'if' blocks that need braces).

[1]: https://codereview.chromium.org/26659003/

Review URL: https://codereview.chromium.org/26995003

git-svn-id: svn://svn.chromium.org/blink/trunk@159669 bbb929c8-8fbe-4397-9dbb-9b2b20218538