tools/gn: another round of fixes to keep bootstrap working

This patch includes metrics_hashes.cc and md5.cc to the base's sources
list to fix the undefined errors as below:

base/metrics/histogram.o: In function
`base::Histogram::Histogram(std::string const&, int, int, base::BucketRanges const*)':
histogram.cc:(.text+0xfc0): undefined reference to `base::HashMetricName(std::string const&)'

BUG=569821
TEST=./tools/gn/bootstrap/bootstrap.py
TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/1534313002

Cr-Original-Commit-Position: refs/heads/master@{#366194}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9608e1467c41eaabab78e68da96a00f475aa25d6

tools/gn/bootstrap/bootstrap.py

diff --git a/tools/gn/bootstrap/bootstrap.py b/tools/gn/bootstrap/bootstrap.py
index 967c80d..38003e3 100755
--- a/tools/gn/bootstrap/bootstrap.py
+++ b/tools/gn/bootstrap/bootstrap.py
@@ -189,6 +189,7 @@
       'base/lazy_instance.cc',
       'base/location.cc',
       'base/logging.cc',
+      'base/md5.cc',
       'base/memory/ref_counted.cc',
       'base/memory/ref_counted_memory.cc',
       'base/memory/singleton.cc',
@@ -202,6 +203,7 @@
       'base/metrics/histogram.cc',
       'base/metrics/histogram_base.cc',
       'base/metrics/histogram_samples.cc',
+      'base/metrics/metrics_hashes.cc',
       'base/metrics/sample_map.cc',
       'base/metrics/sample_vector.cc',
       'base/metrics/sparse_histogram.cc',