trial_token_unittest: Document how to use or update the public key.

content/common/origin_trials/trial_token_unittest.cc

Index: content/common/origin_trials/trial_token_unittest.cc
diff --git a/content/common/origin_trials/trial_token_unittest.cc b/content/common/origin_trials/trial_token_unittest.cc
index 784cd938dd0e1e3d4bfa366c7801258374fd8bc8..3c88134d1a59368ce4eb8ad580615285471f8fbc 100644
--- a/content/common/origin_trials/trial_token_unittest.cc
+++ b/content/common/origin_trials/trial_token_unittest.cc
@@ -28,6 +28,13 @@ namespace {
 //  0x10, 0xac, 0xf9, 0x3a, 0x1c, 0xb8, 0xa9, 0x28, 0x70, 0xd2, 0x9a,
 //  0xd0, 0x0b, 0x59, 0xe1, 0xac, 0x2b, 0xb7, 0xd5, 0xca, 0x1f, 0x64,
 //  0x90, 0x08, 0x8e, 0xa8, 0xe0, 0x56, 0x3a, 0x04, 0xd0
+//
+//  This private key can also be found in tools/origin_trials/eftest.key in
+//  binary form. Please update that if changing the key.
+//
+//  To use this with a real browser, use --origin-trial-public-key with the
+//  public key, base-64-encoded:
+//  --origin-trial-public-key=dRCs+TocuKkocNKa0AtZ4awrt9XKH2SQCI6o4FY6BNA=
 const uint8_t kTestPublicKey[] = {
     0x75, 0x10, 0xac, 0xf9, 0x3a, 0x1c, 0xb8, 0xa9, 0x28, 0x70, 0xd2,
     0x9a, 0xd0, 0x0b, 0x59, 0xe1, 0xac, 0x2b, 0xb7, 0xd5, 0xca, 0x1f,