linux: fix the shared build by excluding cookie_modal_dialog.
linux: fix the shared build by excluding cookie_modal_dialog.

This will have to be reverted of course once the linux-gtk implementation
of CookiePromptModalDialog lands.

Error was: out/Debug/lib.target/chrome/libbrowser.so: undefined reference
to `vtable for CookiePromptModalDialog'

BUG=None
TEST=compiles

Patch from Thiago Farina <thiago.farina@gmail.com>

Review URL: http://codereview.chromium.org/595002

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@38618 0039d316-1c4b-4281-b951-d872f2087c98

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