Make LoadingBehaviorFlag a typed enum and add operators

This change improves the type safety and usability of the
LoadingBehaviorFlag enum by:

1.  Explicitly setting the underlying type to `uint32_t`.
2.  Introducing `operator|` and `operator|=` overloads.

These changes allow for direct bitwise operations on LoadingBehaviorFlag
enum values without requiring explicit static_casts, making the code
in `document_loader.cc` cleaner and less error-prone.

This is a preliminary for synthetic response UMA fix.

Change-Id: I5d2571c89b7f671dd51cf32d1d26db8e2d35fd2d
Bug: 352578800
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6882717
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Commit-Queue: Shunya Shishido <sisidovski@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1506435}