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Previously, document policy class is designed to hold a field for each
feature's policy value. Those fields are supposed to be generated from
document_policy_features.json5. Such design minimizes both speed and
memory overhead but adds complexity to the system by introducing code
generation.

This CL uses array to represent internal feature state, which still
minimizes speed overhead as indexing into array is the same as indexing
into memory, and adds very little memory overhead(Each PolicyValue
instance contains extra information).

Change-Id: I4bc5e9b4383c628018e2605ad896e31b17c9c0e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2003436
Commit-Queue: Charlie Hu <chenleihu@google.com>
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732682}