In doing so, remove use of gurl which causes a dependency on ICU. As
ClipboardRecentContent is used in extensions it is preferable to have
as small a binary as possible and ICU is a large library. An additional
benefits of having an Obj-C implementation with a C++ wrapper is that
the Obj-C implementation can be directly used without introducing C++
in the extension.

BUG=671166, 622743

Review-Url: https://codereview.chromium.org/2782823003
Cr-Commit-Position: refs/heads/master@{#463582}