Clear the PrintContext in WebLocalFrameImpl::Close().

      Also explicitly call PrintContext::EndPrintMode(), rather than calling
it in the destructor. Add a ScopedPrintMode helper class to do that
conveniently.

BUG=716474

Review-Url: https://codereview.chromium.org/2848823005
Cr-Commit-Position: refs/heads/master@{#468882}