Convert some editing layout tests to platform independent

This patch is a preparation of fixing issue 351981.

This patch converts following layout tests for editing to platform independent:
* return-key-in-hidden-field.html:
Use JS test instead of comparing text result.
* copy-in-password-field.html
Don't check editing callbacks, because we don't care selection changes during.
  editing function execution.
* paste-plaintext-user-select-none.html
Convert pixel test to JS test
* select-element-1.html
Convert original test framework to JS test and don't check editing callbacks,
because we don't care selection changes during.
* 4895428-3.html
Convert pixel test to JS test.

BUG=n/a
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/431933002

git-svn-id: svn://svn.chromium.org/blink/trunk@179389 bbb929c8-8fbe-4397-9dbb-9b2b20218538