android webview: Fix generating mac buildfiles while on linux.

The path to android_sdk_tools depends on the host OS at build time,
which is not guaranteed to be the same as the host OS at gyp time on
the WebView build.

BUG=346293
R=benm@chromium.org

Review URL: https://codereview.chromium.org/177723002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252918 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252918