update_engine: Refactor OperationsGenerator into a base class.

This refactor cleans up the interface of the algorithms that generate
the list of rootfs and kernel operations removing the mention of a
Graph from it. The Graph class is only used by the in-place generator
because it requires to keep track of dependencies between operations
reading or writting the same block. The full update generator, using
only REPLACE or REPLACE_BZ doesn't need to use a graph to do that, but
in order to reuse some code, the interface was hacked that way.

This patch now uses two vectors of "AnnotatedOperations", which are
a mere InstallOperation as defined by the .proto file plus a name
used for logging purposes only. Both rootfs and kernel operations
have now the same type on the interface, allowing to share common
functions handling those.

BUG=chromium:331965
TEST=FEATURES=test emerge-link update_engine

Change-Id: I78566bbecb948634b7ecc8d086766ce67a79b43e
Reviewed-on: https://chromium-review.googlesource.com/262281
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
Trybot-Ready: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
diff --git a/delta_performer_unittest.cc b/delta_performer_unittest.cc
index fc2ac7e..e0bd95d 100644
--- a/delta_performer_unittest.cc
+++ b/delta_performer_unittest.cc
@@ -525,7 +525,7 @@
 
     EXPECT_TRUE(payload_config.Validate());
     EXPECT_TRUE(
-        DeltaDiffGenerator::GenerateDeltaUpdateFile(
+        GenerateUpdatePayloadFile(
             payload_config,
             state->delta_path,
             private_key,