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This change adds a "mutable" option to flatbuffer template,
let it using "--gen-mutable" flag to compile flatbuffer with
non-const accessors for mutating FlatBuffers in-place.

Bug: 1149150
Test: Compile flatbuffer see if it generates mutable header
Change-Id: I42ce7dfcac5ee8496db56548391b205d0b056aa3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2534550
Commit-Queue: Fish Lin <linfish@google.com>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827709}