update_engine: port from libxml2 to expat.

This allows us to remove libxml2 from the OS and thereby save a couple
of MB of disk space.

BUG=chromium:293137
TEST=Unit tests pass + manual testing.
CQ-DEPEND=CL:209651

Change-Id: I2469f1862dd7e25dd6684640a755745f09b4db06
Reviewed-on: https://chromium-review.googlesource.com/209770
Reviewed-by: David Zeuthen <zeuthen@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
diff --git a/update_engine.gyp b/update_engine.gyp
index e8e3723..4b09e8c 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -107,7 +107,7 @@
           'libcurl',
           'libmetrics-<(libbase_ver)',
           'libssl',
-          'libxml-2.0',
+          'expat'
         ],
         'deps': ['<@(exported_deps)'],
       },