[DevTools] Network: Fix time delta calculation for N/A values.
[DevTools] Network: Fix time delta calculation for N/A values.


BUG=266717

Review URL: https://chromiumcodereview.appspot.com/22433002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@155698 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