Rename webkit -> blink in Blink .gyp and .gypi files.

Response to request by abarth@ in:
Issue 21130002: Rename EventNames.in to EventInterfaces.in
https://codereview.chromium.org/21130002/

Only changed use in gen directories in Blink; there are other uses in target names (and in Chromium) which I'm not familiar with.

Clean build of chrome yields no gen/webkit dir (only gen/blink).

Didn't change:
Tools/DumpRenderTree/DumpRenderTree.gyp/DumpRenderTree.gyp
...as it's messy and I believe deprecated anyway.

Can check via:
grep -l '/webkit' $(git ls-files '*.gyp' '*.gypi')
...which (post this CL) yields only
DumpRenderTree.gyp

More hits (targets, comments) with:
grep -l 'webkit' $(git ls-files '*.gyp' '*.gypi')

Can check in Code Search via:
"/webkit" package:^chromium$ file:^src/third_party/WebKit/.*.gypi? case:yes
https://code.google.com/p/chromium/codesearch#search/&q=%22DIR)/webkit%22%20package:%5Echromium$%20file:%5Esrc/third_party/WebKit/.*.gypi?&sq=package:chromium&type=cs

Review URL: https://chromiumcodereview.appspot.com/21064005

git-svn-id: svn://svn.chromium.org/blink/trunk@155164 bbb929c8-8fbe-4397-9dbb-9b2b20218538