Move two generic string split functions from sync API to their own API in bas...

Move two generic string split functions from sync API to their own API in base/string_split.

BUG=None 
TEST=base_unittests

Original patch by Thiago Farina
Original Review: http://codereview.chromium.org/464075


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=36774

[tfarina (gmail-do not use), 2009-12-23 18:40:41]

Tim, if something failed on the trybots, please send me a email, then I can look
at the failures and fix it :)

I saw that you fixed some problems on Mac compilation.

[tfarina (gmail-do not use), 2010-01-15 15:01:56]

Tim do you will land this patch?

[Denis Lagno, 2011-03-24 15:49:19]

http://codereview.chromium.org/502074/diff/12001/base/string_split.h
File base/string_split.h (right):

http://codereview.chromium.org/502074/diff/12001/base/string_split.h#newcode17
base/string_split.h:17: std::string* key, std::vector<std::string>* values);
I have concerns about these
First problem is its weird signature:

bool SplitStringIntoKeyValues(
    const std::string& line,
    char key_value_delimiter,
    std::string* key, std::vector<std::string>* values);

contrary to this signature function splits string into single key and single
value.  Probably sometime emptiness of output vector served as error code but
now it has separate error return value.
Ok, that's easy to fix.  But there are other problems:

It accepts empty keys while rejects empty values.  For me it sounds weird: empty
values are common and normal but empty key usually means that something is
broken.
How it handles multiple delimiters like in "key===value=" -- it splits it as
"key" and "value=" -- this looks not great, I prefer "key" and "==value="
because it does not place unnecessary restriction (do not start with delimiter)
on value strings.  And BTW it does not check multiple delimiters in unittests:)

Currently those functions are used only in gaia auth, so I fear to break
something by changing it but in its current form I doubt this split string
functionality is usable by general public (as implied by its presence in base::)