Make base::trace_event::TracedValue use StringPiece where possible.

Make base::trace_event::TracedValue use StringPiece where possible.

These strings wind up being converted into StringPiece before they are passed
to base::Pickle. By exposing this in the TracedValue interface (which is nearly
transparent to callers, since std::string is implicitly convertible), callers
which have other types convertible to StringPiece can call through without a
copy into an std::string.

Committed: https://crrev.com/9abd507a166493b926dc94605baf16305b42d2f7
Cr-Commit-Position: refs/heads/master@{#360341}

[jbroman, 2015-11-17 18:16:53]

jbroman@chromium.org changed reviewers:
+ primiano@chromium.org

[jbroman, 2015-11-17 18:16:53]

I don't see any reason not to push StringPiece to the API here, and it would
permit callers which hold things other than std::string (e.g. Blink after
https://codereview.chromium.org/1419913002) to avoid extra copies.

[Primiano Tucci (use gerrit), 2015-11-17 18:33:36]

primiano@chromium.org changed reviewers:
+ dsinclair@chromium.org

[Primiano Tucci (use gerrit), 2015-11-17 18:33:36]

Hmm this all makes sense to me.
The only think that makes me "hmm" is: I remember that when I wrote this class I
originally used StringPiece, but then for some reason diverted to std::string.
Of course I cannot remember the reason anymore.
So I'd say this: let me think to this with a more fresh mind, tomorrow (emea)
morning. If nothing comes to my mind I'll LG this one, as I cannot see anything
wrong with it. If something will, well I'll let you know.

In the meantime let's see if dsinclair@ has any clue.
This really looks sensible, just wonder what made me change my mind back then.
Probably just something completely unrelated.

Plz ping me if I'll not have responded to this in 24h.

[Primiano Tucci (use gerrit), 2015-11-18 11:48:06]

Nope, even with freshly brewed coffee I can't see a reason not to do it.
LGTM thanks

[jbroman, 2015-11-18 14:54:54]

The CQ bit was checked by jbroman@chromium.org

[commit-bot: I haz the power, 2015-11-18 14:55:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1451943002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1451943002/1

[commit-bot: I haz the power, 2015-11-18 16:19:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-18 16:20:44]

Patchset 1 (id:??) landed as
https://crrev.com/9abd507a166493b926dc94605baf16305b42d2f7
Cr-Commit-Position: refs/heads/master@{#360341}