seems like a simple track matte reveal to me. you can drive the alpha matte with cc glass, custom displacement, distort, fractal, lightning, etc.
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Find (or draw) a suitable image of cracked glass that you can apply to the window in your footage.
Create a new comp with the image in it. Duplicate the image.
On the upper layer, use the Paint tool (set to Write On) to paint in real time over the cracks, from the centre out, trying to mimic the way the crack would be revealed as it cracked. The Paint tool places keyframes for each stroke that can be retimed to adjust the speed.
Once you're done, set Paint on that layer to "Paint On Transparent", and set the lower layer to use the upper layer as an Alpha Track Matte.
Now, you should see the cracked image expand over time, revealed by your paint brush strokes.
Composite this into your scene.