Add nonstd::unique_ptr as a workaround until std::unique_ptr is allowed.

Add nonstd::unique_ptr as a workaround until std::unique_ptr is allowed.

This is a copy of breakpad's standalone scoped_ptr, which has been
renamed to nonstd::unique_ptr, and from which more complicated classes
have been removed. The reset() method has also been tweaked to more
closely match c++11, and an implicit conversion to bool has been added.

BUG=https://code.google.com/p/pdfium/issues/detail?id=55
R=thestig@chromium.org

Committed: https://pdfium.googlesource.com/pdfium/+/29bc87d102b9e36c1e15385756c9fa9a214f3503

[Tom Sepez, 2015-04-17 16:30:09]

tsepez@chromium.org changed reviewers:
+ thestig@chromium.org

[Tom Sepez, 2015-04-17 16:31:03]

Lei, I can't take it, can't take it no more ...

[Lei Zhang, 2015-04-17 19:54:38]

lgtm

[Tom Sepez, 2015-04-17 19:55:48]

Committed patchset #3 (id:40001) manually as
29bc87d102b9e36c1e15385756c9fa9a214f3503 (presubmit successful).