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nacl: Update Migration guide

This change updates the Migration guide to contain more up to date
information about the PPAPI to Web API gaps.

Change-Id: Ic5f952e805a656cebf9540fa9c8301fc7160527e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031939Reviewed-by: Ben Smith <binji@chromium.org>
Commit-Queue: Ovidio de Jesús Ruiz-Henríquez <odejesush@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739174}