Use #if DCHECK_IS_ON() in base/strings/string_piece.*

Use #if DCHECK_IS_ON() in base/strings/string_piece.*

Use #if DCHECK_IS_ON() instead of
(!defined(NDEBUG) || defined(DCHECK_ALWAYS_ON))
in base/strings/string_piece.* because it is
same thing and more succinct and centralized.

Committed: https://crrev.com/570930fc8fdb676b9a60fb4d4a7b029569518da9
Cr-Commit-Position: refs/heads/master@{#353605}

[Olli Raula, 2015-10-12 14:07:05]

olli.raula@intel.com changed reviewers:
+ thestig@chromium.org

[Olli Raula, 2015-10-12 14:07:15]

Could you review?

[Lei Zhang, 2015-10-12 22:00:45]

The CQ bit was checked by thestig@chromium.org

[Lei Zhang, 2015-10-12 22:00:46]

lgtm

[commit-bot: I haz the power, 2015-10-12 22:02:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1328333002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1328333002/1

[commit-bot: I haz the power, 2015-10-12 22:11:06]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-12 22:11:55]

Patchset 1 (id:??) landed as
https://crrev.com/570930fc8fdb676b9a60fb4d4a7b029569518da9
Cr-Commit-Position: refs/heads/master@{#353605}