Remove unnecessary WebBlob constructors

This is just a follow-up fix to apply sof's comments in
https://codereview.chromium.org/588073003/diff/160001/Source/web/WebBlob.cpp#oldcode45

BUG=340522
TBR=tkent

Review URL: https://codereview.chromium.org/618743005

git-svn-id: svn://svn.chromium.org/blink/trunk@183037 bbb929c8-8fbe-4397-9dbb-9b2b20218538