Prepare NetworkPersistenceManager for source map asyncification
Prepare NetworkPersistenceManager for source map asyncification

The NetworkPersistenceManager uses {addBinding} and {removeBinding}
from the Persistence class. These two functions will be async in the
future to support asynchronous source mapping. This CL turns calling
functions into async function themselves and awaits callees.

R=17a2cbc4e33156874a9862b0f530521117e66a3d@chromium.org

Bug: chromium:1032016
Change-Id: Ic820b935a62860e0a2c121ba8225c163e8989afb
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2030176
Reviewed-by: Mathias Bynens <17a2cbc4e33156874a9862b0f530521117e66a3d@chromium.org>
Commit-Queue: Simon Zünd <38944668e9e2ca98ccd0275b53370f90fa55c113@chromium.org>

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