Avoid comparing with a dangling pointers.

ProjectorNavigationThrottleTest and
ProjectorNavigationThrottleTestParameterized were flaky.
They were flaky due to the assumption that a newly allocated
Browser instance will be in a different memory location that a recently
destroyed Browser instance.

The assumption doesn't hold true because
the memory allocator can potentially reuse a recently freed memory
location for new allocations.

Bug: b/275599083
Change-Id: I2e79055860e1748edd6979aa0588ccebb8c10340
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4375108
Commit-Queue: Yilkal Abe <yilkal@chromium.org>
Reviewed-by: Xiqi Ruan <xiqiruan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1123294}