Memory improvements to render_pdfs; better DM pool size defaults

Make SK_DEFAULT_GLOBAL_DISCARDABLE_MEMORY_POOL_SIZE equal to
skia_resource_cache_mb_limit, if that value is >0 (true for some
low-memory Android devices).

render_pdfs test program uses lazy decoding (and the discardable
memory pool).

BUG=skia:2743
R=djsollen@google.com, mtklein@google.com

Author: halcanary@google.com

Review URL: https://codereview.chromium.org/433063002
4 files changed