Step 2 in removing Files API from shuffler:  In this step we use GCS as the output of the _HashPipeline phase, the _SortChunksPipeline phase and as the input to the _MergePipeline phase of the Shuffle.

R=ozarov
DELTA=243 (134 added, 33 deleted, 76 changed)

Revision created by MOE tool push_codebase.
MOE_MIGRATION=7128