cc: Compute picture pile size at construction time.

cc: Compute picture pile size at construction time.

We log picture size in UMA histograms every time we
prepare to draw. In order to make that code faster,
precompute the pile size when it is constructed.

R=jbroman, enne

Committed: https://crrev.com/bf32c2a4319d84bdde05e0a5ea88cb34c2d9f8f5
Cr-Commit-Position: refs/heads/master@{#329773}

[vmpstr, 2015-05-13 22:22:15]

vmpstr@chromium.org changed reviewers:
+ enne@chromium.org, jbroman@chromium.org

[vmpstr, 2015-05-13 22:25:40]

Hi, I was going through some profiling results, and I noticed that this
GetPictureMemoryUsage appears pretty high on the list of hot functions. I think
it's because we're calling it in LTHI::PrepareToDraw to histogram it. 

This patch makes it better? Although I don't really see too much difference on
local telemetry tests (just in using perf).

Any thoughts? Maybe there's a better way to do this, like not calling
GetPictureMemoryUsage in PrepareToDraw, but instead calling it somewhere else?

[enne (OOO), 2015-05-13 22:42:41]

I could be ok with landing this, but I'm not super fond of making edge case code
that much more complicated.

(1) Maybe we could call it less often? Or, only for the first first n frames or
some such?

(2) If we do want to call it always, maybe we should just figure out the memory
during PicturePile::Update or PushPropertiesTo? At that point the recording
source is frozen, and it seems out of place to lazily do it later?

[vmpstr, 2015-05-13 23:17:18]

I did it in the constructor that constructs pile impl out the pile, which
happens during commit. PTAL.

[enne (OOO), 2015-05-13 23:24:29]

lgtm

[vmpstr, 2015-05-13 23:24:52]

The CQ bit was checked by vmpstr@chromium.org

[commit-bot: I haz the power, 2015-05-13 23:25:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1127153006/40001

[commit-bot: I haz the power, 2015-05-14 01:36:37]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-05-14 01:37:23]

Patchset 3 (id:??) landed as
https://crrev.com/bf32c2a4319d84bdde05e0a5ea88cb34c2d9f8f5
Cr-Commit-Position: refs/heads/master@{#329773}