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This change splits out the targets being used to track the gn
migration from  all.gyp into a separate, new file
(gn_migration.gypi), and reworks their structure to

  1) better match the gn_all target in src/BUILD.gn,
  2) identify the targets built by 'all' in gyp that
     are not yet explicitly listed in the gn_all target
     (the new 'add_to_gn_all' target),
  3) identify the targets left to be converted.

The 'add_to_gn_all' targets will simply be moved to gn_all in
a follow-up CL.

Right now these targets will be linux-only, but follow-up CLs
will expand them to the other configurations.

This CL should produce no actual changes in the builds.

R=brettw@chromium.org
BUG=461019
CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:android_chromium_gn_compile_dbg,android_chromium_gn_compile_rel;tryserver.chromium.win:win8_chromium_gn_rel,win8_chromium_gn_dbg;tryserver.chromium.mac:mac_chromium_gn_rel,mac_chromium_gn_dbg

Review URL: https://codereview.chromium.org/962343002

Cr-Commit-Position: refs/heads/master@{#318599}