Cleanup: Better constify some strings in base.

Fix some lint errors along the way.

Review URL: https://codereview.chromium.org/632103004

Cr-Commit-Position: refs/heads/master@{#300407}
diff --git a/base/md5_unittest.cc b/base/md5_unittest.cc
index 3e7f2ad5..8d817e9 100644
--- a/base/md5_unittest.cc
+++ b/base/md5_unittest.cc
@@ -33,7 +33,7 @@
 
 TEST(MD5, MD5SumEmtpyData) {
   MD5Digest digest;
-  const char* data = "";
+  const char data[] = "";
 
   MD5Sum(data, strlen(data), &digest);
 
@@ -50,7 +50,7 @@
 
 TEST(MD5, MD5SumOneByteData) {
   MD5Digest digest;
-  const char* data = "a";
+  const char data[] = "a";
 
   MD5Sum(data, strlen(data), &digest);