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This patch makes file_error::ErrorCode to be an enum class for better
type safety and improving code health. Using forward declaration in
other header files instead of including |file_error.h|.

Change-Id: I53eae14c2972bb792b474d36e4b8c4bc52c09b0f
Reviewed-on: https://chromium-review.googlesource.com/c/1314083Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Zhuoyu Qian <zhuoyu.qian@samsung.com>
Cr-Commit-Position: refs/heads/master@{#606737}