service worker: Remove fetch-* http tests redundant with WPT tests.

Remove the following tests:
- fetch-canvas-tainting.html
- fetch-cors-xhr.html
- fetch-csp.html

These are redundant with corresponding WPT tests. They previously could
not be removed without losing test coverage because the WPT tests were
being skipped due to time outs, see
https://codereview.chromium.org/2805313003/. But those WPT tests are now
passing. A small exception is the WPT fetch-canvas-tainting* tests
flakily timeout, but they are still enabled and mostly passing so we
won't lose test coverage.

These tests were originally written at r179595 and upstreamed to WPT
at https://github.com/web-platform-tests/wpt/commit/b2e3ab765e26436850208dbdcf747c3be75b1999.

Bug: 688116, 373120
Change-Id: Ic2dc2ddc322aae89b1e06e5a16b1ed8c30265d00
Reviewed-on: https://chromium-review.googlesource.com/c/1370236
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615440}
7 files changed