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This CL renames a couple functions in order to try to more accurately reflect what they do:
  BuildCurrentRevision -> _ObtainBuild
  DownloadCurrentBuild -> _DownloadAndUnzipBuild

This CL breaks two sub-functions out of DownloadCurrentBuild:
  _WaitForBuildDownload
  _UnzipAndMoveBuildProducts

Note: Later, I plan to replace _GetBuildArchiveForRevision with fetch_build.FetchBuild, and possibly change _UnzipAndMoveBuildProducts so that it doesn't do unzipping, since unzipping is done in fetch_build.FetchBuild. Either that, or make it so that unzipping isn't done in that function.

BUG=

Review URL: https://codereview.chromium.org/664793002

Cr-Commit-Position: refs/heads/master@{#301029}