[Media] Move MediaPlaybackWhileNotVisible histogram recording callsite

CL 6349498 introduced histogram
"Media.MediaPlaybackWhileNotVisible.InterruptionType". However, we
noticed that placing the HTMLMediaElement::RejectScheduledPlayPromises
was a mistake and resulted in many
MediaPlaybackInterruptionType::kFrameHiddenWhilePlaying events not being
recorded since this method is not always called. This CL moves the
recording to happen right before WebMediaPlayerImpl::Pause is called.

Manually verified that the histogram recording happens as expected using
https://gabrielsanbrito.github.io/media-playback-while-not-visible/.

Bug: 351354996
Change-Id: I085c5a66096e455c92b875cd271bc19be3b5b755
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6763055
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Gabriel Brito <gabrielbrito@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1489874}