Reset viewport to implicit when @viewport is removed.

When m_propertySet was null after cascading @viewport rules (no rules added),
ViewportStyleResolver::resolve would be a no-op so that removing the last
@viewport rule through CSSOM would not have an effect. This is fixed by
re-setting to Implicit when removing the last @viewport rule.

Removed a stray ViewportStyleResolver::resolve call in viewportAsText that was
a no-op before the rest of this change, but resulted in always resetting the
viewport arguments with this fix.

BUG=276812

Review URL: https://chromiumcodereview.appspot.com/23185008

git-svn-id: svn://svn.chromium.org/blink/trunk@156855 bbb929c8-8fbe-4397-9dbb-9b2b20218538