Credential manager: Fixing missing stub declarations.
Credential manager: Fixing missing stub declarations.

https://src.chromium.org/viewvc/blink?view=rev&revision=180040 introduced
the WebCredentialManager interface, but neglected to create default
constructor/destructor implementations. This trivial patch corrects that
oversight.

BUG=400674
TBR=tkent@chromium.org

Review URL: https://codereview.chromium.org/466713002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@180067 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs