Revert of Move gtest/gmock BUILD.gn files to main tree.

Revert of Move gtest/gmock BUILD.gn files to main tree. (https://codereview.chromium.org/219433005/)

Reason for revert:
Causing breakage on the buildbots.

http://build.chromium.org/p/chromium.mac/builders/Mac%20Builder/builds/1429/ is one example.

________ running 'svn update /Volumes/data/b/build/slave/Mac_Builder/build/src --revision 260951 --force --accept theirs-conflict --ignore-externals' in '/Volumes/data/b/build/slave/Mac_Builder/build'
U    /Volumes/data/b/build/slave/Mac_Builder/build/src/build/config/win/BUILD.gn
U    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/item.h
U    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/command_gen.cc
U    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/gn.gyp
U    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/BUILD.gn
A    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/visibility.cc
U    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/builder.h
U    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/functions.cc
A    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/visibility.h
U    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/variables.cc
U    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/function_toolchain.cc
A    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/visibility_unittest.cc
U    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/variables.h
D    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/secondary/testing
U    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/builder.cc
U    /Volumes/data/b/build/slave/Mac_Builder/build/src/tools/gn/target_generator.cc
svn: UUID mismatch: existing directory '/Volumes/data/b/build/slave/Mac_Builder/build/src/testing/gtest' was checked out from a different repository
Sleeping 5.0 seconds and retrying....

Original issue's description:
> Move gtest/gmock BUILD.gn files to main tree.
> 
> Also moves the "unused variable warning" to a config in the build directory so
> it can be shared with outher targets without duplication on the command line.
> 
> BUG=
> R=cjhopman@chromium.org
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=260949

TBR=cjhopman@chromium.org,brettw@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=

[rlarocque, 2014-04-01 22:00:07]

Created Revert of Move gtest/gmock BUILD.gn files to main tree.