Make editing/pasteboard/copy-image-with-alt-text.html independent from initial value of settings

This patch changes the test script, copy-image-with-alt-text.html, independent from initial value of settings SelectionIncludesAltImageText by explicitly set its value.

At the test script created (Blink r146835, http://src.chromium.org/viewvc/blink?view=revision&revision=146835), initial value is false, then it is changed to true at Chromium r191169, http://src.chromium.org/viewvc/chrome?view=revision&revision=191169

BUG=231865
TEST=LayoutTests/editing/pasteboard/copy-image-with-alt-text.html
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/23591023

git-svn-id: svn://svn.chromium.org/blink/trunk@157098 bbb929c8-8fbe-4397-9dbb-9b2b20218538