[M75 merge] Only honor base@target attributes for links and forms, as the spec requires

https://html.spec.whatwg.org/multipage/semantics.html#get-an-element's-target

(cherry picked from commit cb0fb84e7fd14a20ae9528ab0e3c8e8f9de33663)

Bug: 959973, 974197
Test: http/tests/navigation/location-navigate-iframe-with-base-target.html
Change-Id: Id49d355d41b75a9911365da6cb57e895d266fa72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599770
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#657400}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1660967
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/branch-heads/3770@{#1016}
Cr-Branched-From: a9eee1c7c727ef42a15d86e9fa7b77ff0e63840a-refs/heads/master@{#652427}

• third_party/blink/renderer/core/html/html_anchor_element.cc[diff]
• third_party/blink/renderer/core/loader/form_submission.cc[diff]
• third_party/blink/renderer/core/loader/frame_loader.cc[diff]
• third_party/blink/web_tests/http/tests/navigation/location-navigate-iframe-with-base-target-expected.txt[Added - diff]
• third_party/blink/web_tests/http/tests/navigation/location-navigate-iframe-with-base-target.html[Added - diff]
• third_party/blink/web_tests/http/tests/navigation/resources/location-navigate-iframe-with-base-target-iframe.html[Added - diff]