Support more than 9 arguments for ReplaceStringPlaceholders()

Support more than 9 arguments for ReplaceStringPlaceholders()

- support more than 9 arguments ($1, $2, $3, ..., $9 and $10, $11, $12, ...) for ReplaceStringPlaceholders()
- add the unittest named ReplaceStringPlaceholdersMoreThan9Replacements for testing more than 9 arguments for ReplaceStringPlaceholders()

BUG=16963
TEST=string_util_unittest:StringUtilTest.ReplaceStringPlaceholdersMoreThan9Replacements

[haraken.dev_gmail.com, 2011-04-08 13:06:59]

I am trying to fix the bug 16963. 
I found your name in git blame around the function that I modified. 
Could you take a look at it?
Thanks.

[tony, 2011-04-08 16:39:53]

Also, have you signed the CLA?
http://code.google.com/legal/individual-cla-v1.0.html

If so, please add yourself to AUTHORS in this patch as well.

http://codereview.chromium.org/6821001/diff/1/base/string_util.cc
File base/string_util.cc (right):

http://codereview.chromium.org/6821001/diff/1/base/string_util.cc#newcode862
base/string_util.cc:862: ++i;
What happens if we're at the end of a string like "Foo$1"?  Won't we read past
the end of the string?  Can you add a test for this case?