[Tizen] Stop depending on nspr.

The "use_system_nspr" flag has not had any effect since February 2014
when Chromium commit 74ad623 ("Remove use_system_nspr build flag")
removed it from the build system.

Additionally, Chromium now only builds a very small subset of nspr since
commit 9af1b2e0 ("Remove prtypes.h and prcpucfg*.h from
base/third_party/nspr") so building Chromium's version should not affect
performance or cause compatibility problems.
Merge pull request #2619 from rakuco/tizen-remove-nspr-dependency

[Tizen] Stop depending on nspr.