RFR: 8159364: Gtest unit tests does not support PCH

[Erik Helin]

Hi all,

this patch adds support for pre-compiled headers (PCH) to the gtest unit
tests.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8159364

Webrev:
http://cr.openjdk.java.net/~ehelin/8159364/00/

Testing:
- JPRT

Thanks,
Erik