Guard high water cache stats vars by GR_CACHE_STATS

TBR=halcanary@google.com
NOTRY=true

Review URL: https://codereview.chromium.org/729753003
Experimental: NewSkPDFBackend, multipage_pdf_profiler

NewSkPDFBackend in the experimental/ directory, enabled with GYP
setting.

multipage_pdf_profiler executable in experimental/tools/ directory,
for justifying why we need a new SkPDF backend.