mojo: Fix compilation of AssociatedInterfacePtr::associated_group()
mojo: Fix compilation of AssociatedInterfacePtr::associated_group()

AssociatedInterfacePtr::internal_state_ is not a pointer type, so using
-> causes a compilation error.

BUG=573004
TEST=Calling associated_group() compiles

Review URL: https://codereview.chromium.org/1571333003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#368623}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk