Don't serialize size_t in FlattenableHelpers.

sizeof(size_t) is inconsistent between binaries
built for different architectures and values of this
type should be serialized. In this change we replace
size_t with uint32_t in FlattenableHelpers.

Bug: 154941915
Test: Manually
      1. Wihtout this change:
      2. lunch sdk_gphone_x86_64_arm64-userdebug
      3. m && emulator -no-snapshot-load
      4. Open Chrome -> it crashes
      5. Repeat with this change and make sure chrome
         doesn't crash.
Change-Id: I216a9cdbf5db2c28b1ad2c6b7b308714e361abb0