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This CL includes:
- Introduce GooglePlayWebApkInstallDelegate which can bind to Phonesky's
  unpublished PlayInstallService to install WebAPKs.
- WebApkInstaller uses the GooglePlayWebApkInstallDelegate to ask Play to
  install WebAPKs. The changes are behind the finch flag.

The internal CL is: https://chrome-internal-review.googlesource.com/#/c/305912/

BUG=662149

Review-Url: https://codereview.chromium.org/2515293004
Cr-Commit-Position: refs/heads/master@{#438001}