Remove unneeded 'namespace WebCore' instances in Source/web/*.cpp

These were false positives when I was looking for parts of core whose
implementations live in Source/web.

The Geolocation forward declaration is redundant with the one in
WebGeolocationPermissionRequest.h, while the ones in AssertMatchingEnums.cpp
are not needed at all.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/159383002

git-svn-id: svn://svn.chromium.org/blink/trunk@166917 bbb929c8-8fbe-4397-9dbb-9b2b20218538