autofocus: Fix a crash with an autofocus element in a document without browsing context.

ShouldAutofocus() should check existence of the browsing context.
Otherwise, doc.TopFrameOrigin() returns null.

Before crrev.com/695830, ShouldAutofocus() was called only for
rendered elements. That is to say, the document always had
browsing context.

Bug: 1003228
Change-Id: I2a941c34e9707d44869a6d7585dc7fb9f06e3bf4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1800902
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696291}