update_engine: Add ExtentReader

Currently Each operation in DeltaPerformer has the responsibility of
reading from the source device by itself. Adding an
ExtentReader (similar to ExtentWriter) aggregates all these
responsibilities on DeltaPerformer.source_fd_. This simplifies the
design as each operation does not need to invent is own open/read/close
etc. This specially will be used in conjunction with bspatch and puffin.

BUG=chromium:761138
TEST=FEATURES="test" emerge-amd64-generic update_engine;

Change-Id: I2c7a21a02bd1df06bbaa7ebf374a13f654768721
Reviewed-on: https://chromium-review.googlesource.com/651336
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Andrew de los Reyes <adlr@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>
diff --git a/Android.mk b/Android.mk
index 96b4a90..12b6fc0 100644
--- a/Android.mk
+++ b/Android.mk
@@ -127,6 +127,7 @@
     payload_consumer/bzip_extent_writer.cc \
     payload_consumer/delta_performer.cc \
     payload_consumer/download_action.cc \
+    payload_consumer/extent_reader.cc \
     payload_consumer/extent_writer.cc \
     payload_consumer/file_descriptor.cc \
     payload_consumer/file_descriptor_utils.cc \
@@ -924,6 +925,7 @@
     payload_consumer/bzip_extent_writer_unittest.cc \
     payload_consumer/delta_performer_integration_test.cc \
     payload_consumer/delta_performer_unittest.cc \
+    payload_consumer/extent_reader_unittest.cc \
     payload_consumer/extent_writer_unittest.cc \
     payload_consumer/fake_file_descriptor.cc \
     payload_consumer/file_descriptor_utils_unittest.cc \