[GTK] Disable memory sampler on non-Linux system
https://bugs.webkit.org/show_bug.cgi?id=134483

Patch by Ting-Wei Lan <lantw44@gmail.com> on 2014-08-15
Reviewed by Philippe Normand.

Source/WebKit2/Shared/linux/WebMemorySamplerLinux.cpp only works on
Linux because it uses many Linux-specific features, so we should disable
memory sampler on other systems by default.

* Source/cmake/OptionsGTK.cmake:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@172623 268f45cc-cd09-0410-ab3c-d52691b4dbfc