Change preference APIs to take std::string instead of const char*.

The string can't be NULL, and most callers have std::strings already, so this
makes the intent clearer and avoid unnecessary switch between std::string and
const char*. Right now, the prefs are stored in char*, then converted to
strings, then passed again as char*, only to be converted again, which
creates the unnecessary temporary strings.

For example, PrefValueStore::OnPrefValueChanged passes |key|, which is a
string, as a const char* to PrefValueStore::NotifyPrefChanged, which then
passes it to PrefNotifierImpl::OnPreferenceChanged as a string again.

Our tests show that this change reduces on average by ~9% the number of total
allocations in the browser process.

Notes:
- The underlying pref map in the PrefRegistry is based on string keys and
thus any call to it, e.g. find(), needs to convert to string anyways at the
bottom of the implementation so might as well force the string to be passed
into the API originally.
- A strict char* API, i.e. all the way into the pref map with a custom
comparator/etc., would make memory harder to manage since we can't strictly
force the caller to pass only pointers to constants in .rodata and would
thus require explicit memory management.
- Ideally, we would need a hybrid string class that manages the memory on
demand. That way, it can point to hard coded strings with the memory
management disabled or allocate a buffer that it will manage, to get the
best of both worlds.

BUG=438215

Review URL: https://codereview.chromium.org/753603002

Cr-Commit-Position: refs/heads/master@{#306514}

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