We compile chromium with clang flag -Wglobal-constructors in our
project. We have following compilation error during compilation
of user_input_tracker.cc:

../../chrome/browser/page_load_metrics/user_input_tracker.cc:27:11:
error: declaration requires a global constructor [-Werror,-Wglobal-constructors]
const int kRateLimitClampMillis = (kOldestAllowedEventAgeSeconds * 1000) /

In order to get rid of this global constructor I recommend to declare
static class variable UserInputTracker::kMaxTrackedEvents and global
variable kRateLimitClampMillis as constexpr which will be expanded to
numerical value during compilation.

BUG=686669

Review-Url: https://codereview.chromium.org/2662803002
Cr-Commit-Position: refs/heads/master@{#447000}