[BackgroundFetch] Weed out instances of IconDefinition

from the codebase, now that we're using Manifest::ImageResource instead.

Bug: 860507
Change-Id: Ie8ac76ebe4e4ed1ab8a6b6a090dbf05b28d17f5c
TBR: peter
Reviewed-on: https://chromium-review.googlesource.com/1146656
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577480}

content/common/background_fetch/background_fetch_types.typemap

@@ -15,5 +15,4 @@ type_mappings = [
   "blink.mojom.BackgroundFetchOptions=content::BackgroundFetchOptions",
   "blink.mojom.BackgroundFetchRegistration=content::BackgroundFetchRegistration",
   "content.mojom.BackgroundFetchSettledFetch=::content::BackgroundFetchSettledFetch",
-  "blink.mojom.IconDefinition=content::IconDefinition",
 ]