Fix "favicon_core" target on iOS.
Fix "favicon_core" target on iOS.

FallbackIconService depends on gfx::Canvas that is not implemented
on iOS, so remove components/favicon/core/fallback_icon_service.cc
from the build on iOS.

The compilation was not broken before because favicon_core is a
static_library and no symbols of fallback_icon_service.o was used
on iOS and the file was dropped at link time.

Remove incorrect file exclusion in "favicon_content" target.

BUG=None

Review URL: https://codereview.chromium.org/1413373008

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#358081}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk