Send SkPicture deletion message lazily.

Send SkPicture deletion message lazily.

If no one has read the picture's unique ID, there's no point invalidating it.

This is the same trick we pull with SkPixelRefs.

Before:
  26M	1	1.49µs	1.6µs	1.77µs	6.25µs	42%	picture_overhead_draw
  13M	32	742ns	749ns	756ns	823ns	2%	picture_overhead_nodraw

After:
  26M	1	1.27µs	1.33µs	1.49µs	5.51µs	45%	picture_overhead_draw
  14M	43	677ns	680ns	681ns	701ns	1%	picture_overhead_nodraw

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/e35268ef4cdaa29600671cdf9d0ebb43b622d1a3

[mtklein, 2015-04-07 11:58:27]

mtklein@google.com changed reviewers:
+ mtklein@google.com, reed@google.com

[mtklein, 2015-04-07 12:29:05]

The CQ bit was checked by mtklein@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-04-07 12:29:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1061283002/1

[commit-bot: I haz the power, 2015-04-07 12:29:17]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-04-07 12:29:20]

Presubmit check for 1061283002-1 failed and returned exit status 1.

Running presubmit commit checks ...

** Presubmit ERRORS **
Since the CL is editing public API, you must have an LGTM from one of:
('reed@chromium.org', 'reed@google.com', 'bsalomon@chromium.org',
'bsalomon@google.com', 'djsollen@chromium.org', 'djsollen@google.com')

[mtklein, 2015-04-07 13:17:37]

The CQ bit was checked by mtklein@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-04-07 13:17:53]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1061283002/1

[commit-bot: I haz the power, 2015-04-07 13:17:57]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-04-07 13:17:57]

Presubmit check for 1061283002-1 failed and returned exit status 1.

Running presubmit commit checks ...

** Presubmit ERRORS **
Since the CL is editing public API, you must have an LGTM from one of:
('reed@chromium.org', 'reed@google.com', 'bsalomon@chromium.org',
'bsalomon@google.com', 'djsollen@chromium.org', 'djsollen@google.com')

[reed1, 2015-04-07 13:19:31]

The actual (lazy) computation for the uniqueID... is it capturable in a helper
function or something?

lgtm

[mtklein, 2015-04-07 13:21:24]

On 2015/04/07 13:19:31, reed1 wrote:
> The actual (lazy) computation for the uniqueID... is it capturable in a helper
> function or something?
> 
> lgtm

Hmm.  We've used it twice so far, but it's not generalizable between the two
because of the extra bit we track for SkPixelRefs, unless we want to burn a bit
here.  I agree this wants to be written once, though, so I'll give merging them
a shot in a follow up.  We can certainly afford to burn a bit.

[mtklein, 2015-04-07 13:21:32]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-04-07 13:21:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1061283002/1

[commit-bot: I haz the power, 2015-04-07 13:34:09]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/e35268ef4cdaa29600671cdf9d0ebb43b622d1a3