SkPDF: add canon assert before adding code that might break it

SkPDF: add canon assert before adding code that might break it

Motivation: We can write subsets (by page) of pdf documents (but this
in't yet exposed in the public API), but it is a bad idea to mix pages
from multiple documents (de-duping will break).  This assert verifies
that we don't do this by accident in the future.

BUG=skia:3585
Committed: https://skia.googlesource.com/skia/+/26b5d15dab81883fac05a2d3685ca6e3a3459678

[hal.canary, 2015-03-25 14:59:30]

The CQ bit was checked by halcanary@google.com

[commit-bot: I haz the power, 2015-03-25 14:59:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1037573005/1

[commit-bot: I haz the power, 2015-03-25 14:59:40]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2015-03-25 20:59 UTC

[hal.canary, 2015-03-25 14:59:41]

halcanary@google.com changed reviewers:
+ mtklein@google.com

[hal.canary, 2015-03-25 14:59:42]

ptal

[mtklein, 2015-03-25 15:14:40]

The CQ bit was checked by mtklein@google.com

[mtklein, 2015-03-25 15:14:40]

lgtm

[commit-bot: I haz the power, 2015-03-25 15:14:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1037573005/20001

[commit-bot: I haz the power, 2015-03-25 15:38:08]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/26b5d15dab81883fac05a2d3685ca6e3a3459678