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Specifically add BUILD.gn boilerplate (generated with a local script)
for:

 - Auto-generating Polymer 3 versions for each file.
 - Type checking auto-generated files.

Known issues:
 1) Generated code is not yet included in the build. This will happen
    at later stages along with porting tests to Polymer 3.
 2) js_type_check() targets are commented out, because JS type
    checking is not passing yet.
 3) Some polymer_modulize(), js_modulize() target are commented out
    because they cause the auto-generation tools to throw errors.

2 and 3 will be fixed in follow up CLs, either by modifying the
auto-generation tools, or by modifying the source code.

Bug: 1026426
Change-Id: I6e90ec6af32ce59242795bb7d5e8aa6486f0de55
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2021191
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737050}