[chromecast] Reenable v8_use_external_startup_data, disable include_both_v8_snapshots.

We had disabled v8_use_external_startup_data to work around
b/419216998, but
https://chromium-review.googlesource.com/c/chromium/src/+/6602174
made a change that broke builds where v8_use_external_startup_data is
false but include_both_v8_snapshots is true (b/422518569).

Simply restoring v8_use_external_startup_data still regresses to the
original issue, I think because we are only bundling
snapshot_blob_64.bin in the apk, rather than "both" snapshots as the
buildflag describes. Disabling include_both_v8_snapshots on its own also
works, but leaves cast_browser more divergent from other chromium
Android builds than it would be if we used v8_use_external_startup_data.

This combination of buildflags seems best for now, though it may be
worth looking into better protections against this kind of regression:

  * Assert that include_both_v8_snapshots is false if
    v8_use_external_startup_data is false, or make C++ code that checks
    the buildflags check the latter before checking the former
  * Explicitly setting both buildflags in android.gni, so changes to
    defaults in e.g. v8_context_snapshot.gni  don't affect cast_browser
    unintentionally
  * Introduce some kind of browser test that runs on Android
    labs/emulators in chromium infra to verify that the browser can
    start up a web page, which would fail fast if loading the v8
    snapshots fails

Bug: 422518569
Bug: 421165374
Test: build cast_browser_aar, build mediashell arm64 release apk
Test: cast YouTube to Pixel Tablet (uses arm64 abi)
Change-Id: I175d54babb1b7851f2cb64ad28a2b440550affde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6622101
Auto-Submit: Simeon Anfinrud <sanfin@chromium.org>
Reviewed-by: Sandeep Vijayasekar <sandv@google.com>
Commit-Queue: Sandeep Vijayasekar <sandv@google.com>
Cr-Commit-Position: refs/heads/main@{#1469702}