IndexedDB: Timeout transactions if front-end is unresponsive

Introduce a fixed time-out that limits how long a
transaction will wait after it handles the last request
before it gives up and aborts. This is to prevent "wedged"
renderers (not crashed but not turning the event loop, etc)
from holding up the transaction queue since they never
issue a another request or ask to commit.

(TBR for new file entry in gypi)

BUG=318992
R=dgrogam
TBR=jam

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=239543

Review URL: https://codereview.chromium.org/67173010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239956 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239956