[Password Generation] Add comment about keeping |kDefaultPasswordLength| in

sync with its server counter part.

The auto-fill crowd-sourcing server tries to predicts password length requirements for a form in
the classifications pipeline. The server predicts password lengths only for forms which have
lengths smaller than the default password length used by Chrome hence the
password requirements classifier needs to keep track of the default length of the
passwords generated by Chrome, which needs to be kept in sync with the constant on the
client to avoid conflicts.

Change-Id: I7d8ab63294010989d2996928aa10d25a0d383f7c
Reviewed-on: https://chromium-review.googlesource.com/1230064
Commit-Queue: Swapnil Raj <swapnilraj@google.com>
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Christos Froussios <cfroussios@chromium.org>
Reviewed-by: Vaclav Brozek <vabr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592490}

components/autofill/core/browser/password_generator.cc

@@ -16,6 +16,8 @@
 
 namespace autofill {
 
+// The default length for a generated password. If you intend to change this
+// constant, make sure to update the corresponding constant on the server.
 const uint32_t kDefaultPasswordLength = 15;
 
 namespace {