UM: Dump the EvaluationContext when evaluation of a policy fails.

This gives the log reader a complete picture of the state of variables
at the time when a policy evaluation has failed.

We slightly revise the JSON output of EvaluationContext::DumpContext()
to remove trailing whitespaces (newline, etc).

BUG=chromium:387937
TEST=Observed context dumping during unit testing.

Change-Id: I33712a896225f3be97bc313468f89f9cd724673b
Reviewed-on: https://chromium-review.googlesource.com/205896
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
diff --git a/update_manager/evaluation_context_unittest.cc b/update_manager/evaluation_context_unittest.cc
index ab07a12..eafb219 100644
--- a/update_manager/evaluation_context_unittest.cc
+++ b/update_manager/evaluation_context_unittest.cc
@@ -400,7 +400,7 @@
             "      \"fake_int\": \"42\",\n"
             "      \"fake_poll\": \"Hello \\\"world\\\"!\"\n"
             "   }\n"
-            "}\n",
+            "}",
             eval_ctx_->DumpContext());
 }