ContentCapture: fix a crash

The WebContentCaptureClient could be set to null when
RenderFrameImpl::WasShown() is called, instead of, accessing
content_capture_manager_ directly, GetContentCaptureManager()
which check WebContentCaptureClient availability should be used.

This patch also renames GetContentCaptureManager() to
GetOrResetContentCaptureManager()

Bug: 1312504
Change-Id: I855cb90ac11d3d13729b7e4d20bfc56af5bce5bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3572581
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Michael Bai <michaelbai@chromium.org>
Cr-Commit-Position: refs/heads/main@{#992764}