Remove PersistentPrefStore::MarkNeedsEmptyValue.

Remove PersistentPrefStore::MarkNeedsEmptyValue.

Instead of this hack, JsonPrefStore now cleans up less aggressively and
correctly distinguishes between not-set and empty values.

BUG=None
TEST=Passes unit tests.

[Mattias Nissler (ping if slow), 2013-01-28 14:14:16]

FWIW, here's an attempt at removing MarkNeedsEmptyValue. It changes semantics of
dictionary and list prefs persistence though, so I thought I'd fire trybots to
see whether this results in test failure. Probably needs a bit of code auditing
as well to make sure we don't break anything. In any case, I think this is saner
than MarkNeedsEmptyValue + DeepCopyWithoutEmptyChildren, and we should probably
just fix up all client code that has come to rely on that weirdness.

[gab, 2013-11-27 13:46:23]

On 2013/01/28 14:14:16, Mattias Nissler wrote:
> FWIW, here's an attempt at removing MarkNeedsEmptyValue. It changes semantics
of
> dictionary and list prefs persistence though, so I thought I'd fire trybots to
> see whether this results in test failure. Probably needs a bit of code
auditing
> as well to make sure we don't break anything. In any case, I think this is
saner
> than MarkNeedsEmptyValue + DeepCopyWithoutEmptyChildren, and we should
probably
> just fix up all client code that has come to rely on that weirdness.

Moved to https://codereview.chromium.org/81183005 and committed there. Closing
this issue.