weblayer: throw exception before crash

We don't support synchronously exiting fullscreen when fullscreen
is entered. The code currently CHECKs in the native side. This
patch throws an exception in hopes of better identifying why
the java side is synchronously exiting. It doesn't seem the
embedder has a synchronous path that exits, but perhaps it's
because of a config change/retain issue?

BUG=1142101
TEST=none

Change-Id: Ia41dd9018c210a0b300223329a650cb0651030b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626191
Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843075}