Avoid string copy of url.scheme() in ChildProcessSecurityPolicyImpl.

In every method of ChildProcessSecurityPolicyImpl, this CL makes
url.scheme() being called only once.

BUG=None

Review-Url: https://codereview.chromium.org/2966203002
Cr-Commit-Position: refs/heads/master@{#485095}
1 file changed