platform2: Switch over to using base64 functions from libchromeos

Replaced existing implementations of Base64Encode/Base64Decode
with the functions from libchromeos, which were added as part
of an earlier change (see CL:247690).

BUG=None
TEST=`FEATURES=test emerge-link cryptohome debugd metrics privetd update_engine`

Change-Id: I8cec677ce2c2fd3b97ca2228d35c2cf5cd133f4c
Reviewed-on: https://chromium-review.googlesource.com/247792
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/metrics/uploader/metrics_log_base_unittest.cc b/metrics/uploader/metrics_log_base_unittest.cc
index fe02fea..01a0273 100644
--- a/metrics/uploader/metrics_log_base_unittest.cc
+++ b/metrics/uploader/metrics_log_base_unittest.cc
@@ -6,7 +6,6 @@
 
 #include <string>
 
-#include <base/base64.h>
 #include <base/metrics/bucket_ranges.h>
 #include <base/metrics/sample_vector.h>
 #include <gtest/gtest.h>