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During initial population of the extensions and apps lists, addItem()
was called N times, which internally performs a linear search N times,
as well as triggres splice() notifications that are unnecessary.

As revealed by DevTools (for a total of 30 extensions and apps):
 - before: the N calls to addItem() were taking ~22ms total
 - after: The initialization of the lists takes ~5ms total 

Bug: 764126
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: If66d752cd3e7ac4b4ecd01ea4964ca6fd8d8f0db
Reviewed-on: https://chromium-review.googlesource.com/746967Reviewed-by: Dave Schuyler <dschuyler@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512989}