JSONStringValueSerializer: Constructor argument must not be null.

JSONStringValueSerializer: Constructor argument must not be null.

Previously, this class would fail gracefully if given a nullptr. Now it
has undefined behaviour. Dealing with null isn't necessary as no caller
passes nullptr here, and in general, pass-by-result style functions do
not expect null pointers. Dealing with nullptr makes my follow-up work
messy, so I am just removing it.

Also, moved constructor implementation to .cc file, and properly
documented the lifetime constraints on |json_string|.

BUG=455068
Committed: https://crrev.com/4ea5490a5e7b248389249756f132403b61a6944a
Cr-Commit-Position: refs/heads/master@{#314703}

[Matt Giuca, 2015-02-04 03:52:14]

mgiuca@chromium.org changed reviewers:
+ rvargas@chromium.org

[Matt Giuca, 2015-02-04 03:52:15]

Hi Ricardo,

This is a precursor to https://codereview.chromium.org/895913004 (it is much
easier if I don't have to deal with a "null StringPiece", and as I explained in
the CL description, I don't think dealing with null is necessary here).

[rvargas (doing something else), 2015-02-04 21:05:11]

lgtm

[Matt Giuca, 2015-02-04 23:11:02]

The CQ bit was checked by mgiuca@chromium.org

[commit-bot: I haz the power, 2015-02-04 23:12:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/889303004/1

[commit-bot: I haz the power, 2015-02-05 01:22:52]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-05 01:23:56]

Patchset 1 (id:??) landed as
https://crrev.com/4ea5490a5e7b248389249756f132403b61a6944a
Cr-Commit-Position: refs/heads/master@{#314703}