Flush GrContext between benchmark draw loops

Flush GrContext between benchmark draw loops

This change updates a small subset of benchmarks to flush the GrContext
between draw loops (specifically SKP benchmarks, SampleApp, and the
warmup in visualbench). This helps improve timing accuracy by not
allowing the gpu to batch across draw boundaries in the affected
benchmarks.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/e6d2024c689199b09df0d7048fc5252179d52aff

[Chris Dalton, 2015-10-26 19:24:28]

cdalton@nvidia.com changed reviewers:
+ bsalomon@google.com, joshualitt@google.com

[joshualitt, 2015-10-26 19:39:20]

On 2015/10/26 19:24:29, Chris Dalton wrote:

lgtm

[Chris Dalton, 2015-10-26 20:03:36]

The CQ bit was checked by cdalton@nvidia.com

[commit-bot: I haz the power, 2015-10-26 20:03:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1427533002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1427533002/1

[commit-bot: I haz the power, 2015-10-26 20:04:38]

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

[commit-bot: I haz the power, 2015-10-26 20:04:38]

Try jobs failed on following builders:
  Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Debug-Trybot on client.skia (JOB_FAILED,
http://build.chromium.org/p/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX2...)

[Chris Dalton, 2015-10-26 20:34:17]

The CQ bit was checked by cdalton@nvidia.com

[Chris Dalton, 2015-10-26 20:34:18]

The patchset sent to the CQ was uploaded after l-g-t-m from
joshualitt@google.com
Link to the patchset: https://codereview.chromium.org/1427533002/#ps20001
(title: "Check for SK_SUPPORT_GPU")

[commit-bot: I haz the power, 2015-10-26 20:34:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1427533002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1427533002/20001

[commit-bot: I haz the power, 2015-10-26 20:45:32]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/e6d2024c689199b09df0d7048fc5252179d52aff