Changes needed for MacOS X 10.4 support.

Add "support_macosx_10_4" option to common.gypi that causes it to change deployment target, and define a new preprocessor symbol on the Mac build.  Setting this flag to true is harmless on non Mac builds and has no effect.
Make various changes to source files where they modify their behavior in the presence of the new preprocessor symbol to become 10.4 compatible.
Review URL: http://codereview.chromium.org/201122

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@26285 0039d316-1c4b-4281-b951-d872f2087c98