Add mac_write_linker_maps gyp define.

With GYP_DEFINES=mac_write_linker_maps=1, ld64 will be instructed to write
.map files, which can then be visualized to get an idea of binary size.

(Works best in a static library build.)

BUG=none

Review URL: https://codereview.chromium.org/116443008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242693 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242693