Supply separate flags for onBuildTileIndex

Supply separate flags for onBuildTileIndex

Since png and jpeg's implementations of onBuildTileIndex rely on
modifications to their underlying libraries, rather than whether we are
running on Android, use separate flags that can be disabled
independently.

This will allow us to easily turn off the feature. It also is a step
towards building and running on other platforms for testing (e.g.
valgrind/ASAN to find memory leaks etc).

Committed: https://skia.googlesource.com/skia/+/8e48ed51baf0099d0a279474f6fc01a3ab857d60

[scroggo, 2015-10-13 20:34:50]

scroggo@google.com changed reviewers:
+ msarett@google.com

[scroggo, 2015-10-13 20:34:50]

This may make the switchover a little easier. On the other hand, we could just
wait and delete the code when it's time. This version also allows us to try to
debug on valgrind/asan if we want to try and figure out forked libpng's
problems...

[msarett, 2015-10-13 22:04:34]

lgtm

[scroggo, 2015-10-14 14:23:49]

The CQ bit was checked by scroggo@google.com

[commit-bot: I haz the power, 2015-10-14 14:24:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1401283003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1401283003/1

[commit-bot: I haz the power, 2015-10-14 14:54:36]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/8e48ed51baf0099d0a279474f6fc01a3ab857d60