[fuchsia] Reland "Generate BUILD.gn from SDK metadata..."

Reland of CL https://chromium-review.googlesource.com/c/chromium/src/+/1241842.
(Confirmed that the code builds on Cast builders, and manually examined
all references to targets under //third_party/fuchsia-sdk to verify
that all FIDL targets have been modified to reference the new sdk/
subdir.)

This script uses the metadata JSON files in the SDK to generate a
BUILD.gn in //third_party/fuchsia-sdk/sdk. The build file is
regenerated whenever a new SDK is downloaded using "gclient sync".

Only FIDL libraries are migrated for now. Additional metadata types can
be migrated in followup CLs.

TBR=scottmg@chromium.org
TBR=sky@chromium.org

Bug: 888753
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I7ec714982cdfbb6f6ccd8fad3cd7cbb48af4eb1c
Reviewed-on: https://chromium-review.googlesource.com/1246681
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#594419}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 386fa9a70028f97ce511bc7bf84e5ce96e47f926
3 files changed