Split default and user-supplied event tracers.

Split default and user-supplied event tracers.

Chrome's got a test scenario where setting the user-supplied tracer is racing with a concurrent use of the default tracer.

BUG=chromium:437044
No public API changes.
TBR=reed@google.com

Committed: https://skia.googlesource.com/skia/+/59bcfaed660369da2a08fc3ad1ec2799b646f3e1

[mtklein_C, 2015-04-21 14:30:07]

mtklein@chromium.org changed reviewers:
+ glider@chromium.org, humper@google.com

[mtklein, 2015-04-21 14:34:57]

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

[commit-bot: I haz the power, 2015-04-21 14:35:16]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1099123002/60001

[commit-bot: I haz the power, 2015-04-21 14:44:54]

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

[commit-bot: I haz the power, 2015-04-21 14:44:56]

Dry run: This issue passed the CQ dry run.

[humper, 2015-04-21 15:39:39]

On 2015/04/21 14:44:56, I haz the power (commit-bot) wrote:
> Dry run: This issue passed the CQ dry run.

lgtm

[mtklein, 2015-04-21 15:47:33]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-04-21 15:47:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1099123002/60001

[Alexander Potapenko, 2015-04-21 15:55:38]

https://codereview.chromium.org/1099123002/diff/60001/src/utils/SkEventTracer...
File src/utils/SkEventTracer.cpp (right):

https://codereview.chromium.org/1099123002/diff/60001/src/utils/SkEventTracer...
src/utils/SkEventTracer.cpp:55: if (SkEventTracer* tracer =
sk_atomic_load(&gUserTracer, sk_memory_order_relaxed)) {
Since you're dereferincing this pointer, you need an acquire-load/release-store
pair from the C++ memory model POV.

[commit-bot: I haz the power, 2015-04-21 16:38:07]

Committed patchset #4 (id:60001) as
https://skia.googlesource.com/skia/+/59bcfaed660369da2a08fc3ad1ec2799b646f3e1