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Adds two new headers intended for use by files that are compiled under
both EG1 and EG2.  These new headers handle importing the appropriate EG
headers, consolidating this boilerplate code into a single location.

Also consolidates setting eDO include dirs in a single location.

BUG=930859

Change-Id: I02141ea18efc38ef534ed043d61e984da1096d64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1508578Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638767}