CSS Calc: Ensure non-negative properties stay non-negative when using calc()

The m_isNonNegative field in platform calcs was never used. This change
adds the necessary clamping for non-negative calc values.

BUG=423456

Review URL: https://codereview.chromium.org/670653002

git-svn-id: svn://svn.chromium.org/blink/trunk@184062 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[ServiceWorker] Don't taint the CORS response image from the ServiceWorker

This change depends on https://codereview.chromium.org/653373002/ and https://codereview.chromium.org/659573006/


BUG=424473
TEST=http/tests/serviceworker/resources/fetch-canvas-tainting-iframe.html

Review URL: https://codereview.chromium.org/658703002

git-svn-id: svn://svn.chromium.org/blink/trunk@184063 bbb929c8-8fbe-4397-9dbb-9b2b20218538