media: Reenable EncryptedMedia* browser tests

In the previous CL [1] for EncryptedMediaSupportedTypes* browser tests
we disabled WhatsNew page which seems to be stopping the elevated test
flakiness. This CL disables WhatsNew page for EncryptedMedia* browser
tests and reenables disabled tests to resume test coverage.

Note that previously some tests were also flaky, but at a very low rate.
This might be related to how NavigateToUrl() works. See more detailed
discussion in the bug.

This should also stop ongoing flakiness on Windows, e.g. [2].

[1] https://chromium-review.googlesource.com/c/chromium/src/+/3141782
[2] https://analysis.chromium.org/p/chromium/flake-portal/flakes/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVyUgsSBUZsYWtlIkdjaHJvbWl1bUBicm93c2VyX3Rlc3RzQEVuY3J5cHRlZE1lZGlhVGVzdC5QbGF5YmFja19BdWRpb0NsZWFyVmlkZW9fV2ViTQw

Bug: 1243903
Test: Reenable tests
Change-Id: Iaa0b69fa46a6d3a6e1b30e078389ba0c70d0d6a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3150033
Reviewed-by: John Rummell <jrummell@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#919444}