android webview envsetup: Set android_ndk_root and android_sdk_version to dummy values.

They shouldn't be used in a webview build, so make that more obvious.

BUG=330631

Review URL: https://codereview.chromium.org/167193002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251696 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251696