Enable ScalableIph based on iph feature flag state

- Enable ScalableIph based on whether an iph feature flag is enabled or
  not.
- This CL enables kScalableIph by default. kScalableIph flag will serve
  as an emergency turn off switch.

Bug: b:330623311
Fixed: b:330623311
Test: browser_tests --gtest_filter=*ScalableIph*
Change-Id: I15163d8ceb773f038606109d543a8856abbef5a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5385278
Reviewed-by: Li Lin <llin@chromium.org>
Commit-Queue: Yuki Awano <yawano@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1278003}