Clean up SetSrc on HTMLImageElement and HTMLSourceElement

The SetSrc() methods with a USVStringOrTrustedURL argument appears to be
unused. Likewise the Src() getter on HTMLImageElement.

Since the SetSrc() method which takes a String doesn't offer much over
just using setAttribute(kSrcAttr, ...), convert users of that and remove
that too.

Change-Id: I3fa3621f030321442ee01b752b7e5ea4b04e9cac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1593259
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#656344}