update_engine: Introduce FilesystemInterface abstraction.

The interaction with the filesystem in the payload generation process
is hard-coded in several places, making it hard to mock out or use a
different filesystem like squashfs for delta generation. For example,
the metadata, regular file data and non-file data are handled by three
different functions in a similar way, but with different code.

This patch introcudes a filesystem abstraction to map files or
pseudo-files (like the metadata, free-space, etc) into the same interface.
The interface includes three implementations: for parsing ext2 filesystems
using ext2fs (already used by the metadata parsing but not by the file
data processing), a raw one for monolitic partitions like the kernel
and a fake one used for testing without requiring to build/parse a real
ext2 filesystem.

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

Change-Id: I1e14cf8f3883c8e9a1d471c8193c8da60776aa7c
Reviewed-on: https://chromium-review.googlesource.com/275803
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>

test_utils.h

diff --git a/test_utils.h b/test_utils.h
index dddd563..5453d27 100644
--- a/test_utils.h
+++ b/test_utils.h
@@ -17,6 +17,7 @@
 #include <vector>
 
 #include <base/callback.h>
+#include <base/files/file_path.h>
 #include <glib-object.h>
 #include <gtest/gtest.h>
 
@@ -209,8 +210,11 @@
 // Frees a GValue object and its allocated resources.
 void GValueFree(gpointer arg);
 
-}  // namespace test_utils
+// Returns the path where the build artifacts are stored. This is the directory
+// where the unittest executable is being run from.
+base::FilePath GetBuildArtifactsPath();
 
+}  // namespace test_utils
 
 // Useful actions for test. These need to be defined in the
 // chromeos_update_engine namespace.