Thread through a flag to force SkPicture::playback() when recording subpictures.

This makes nanobench picture recording benchmarks somewhat useful again,
as opposed to all taking about 5us to run no matter the content.

ATTN Sheriff: this will probably trigger perf.skia.org alerts.

BUG=skia:

Review URL: https://codereview.chromium.org/1219873002
diff --git a/src/core/SkPictureRecorder.cpp b/src/core/SkPictureRecorder.cpp
index c110e0a..877314c 100644
--- a/src/core/SkPictureRecorder.cpp
+++ b/src/core/SkPictureRecorder.cpp
@@ -38,7 +38,10 @@
     if (!fRecord) {
         fRecord.reset(SkNEW(SkRecord));
     }
-    fRecorder->reset(fRecord.get(), cullRect, &fMiniRecorder);
+    SkRecorder::DrawPictureMode dpm = (recordFlags & kPlaybackDrawPicture_RecordFlag)
+        ? SkRecorder::Playback_DrawPictureMode
+        : SkRecorder::Record_DrawPictureMode;
+    fRecorder->reset(fRecord.get(), cullRect, dpm, &fMiniRecorder);
     fActivelyRecording = true;
     return this->getRecordingCanvas();
 }