IndexedDB: Const sprinkles, eschew nested ternary ops, whitespace

Miscellaneous tidying, no functional changes.

Review URL: https://codereview.chromium.org/102403003

git-svn-id: svn://svn.chromium.org/blink/trunk@163131 bbb929c8-8fbe-4397-9dbb-9b2b20218538