Continue buffering paused HTML5 video in background (or unfocused) Safari tab

Just because Safari and Chrome are running, doesn't mean that they have a window open. You can either test for open windows as Mark points out or simply issue the reopen command which opens a new window if one does not exist.

tell application "Google Chrome"
    if not (exists window 1) then reopen
    -- insert your code 
end tell

tell application "Safari"
    if not (exists document 1) then reopen
    -- insert your code 
end tell

EDIT

tell application "Safari"
    if not (exists document 1) then return
    set theURL to URL of current tab of window 1
    close current tab of window 1
end tell

tell application "Google Chrome"
    if not (exists window 1) then reopen
    set URL of active tab of window 1 to theURL
end tell

Window > Show Next Tab

Should display the keyboard shortcut to switch between open tabs, I have mine custom set but I believe the default keyboard shortcut is :

Control+-->

However if you want a specific tab to be populated corresponding to the number you can set custom keyboard shortcuts within Automator or go Ian's route

I was able to reproduce your desired results with Automator in conjunction with the AppleScript you provided :

Automator Walkthrough :

Follow the link to my Discussion

The general parameters are the same and I'm sure you smart enough to figure it out.

1. Services Receive : NO INPUT
2. In : Select Safari so this service isn't available outside the application
3. Drag and Drop the 'Run AppleScript' action to the workflow
4. Include the AppleScript code you provided
5. Save the service - I saved mine in increments
6. System Preferences > Keyboard > Shortcuts > Services > General > Safari Tab 3 (whatever you named yours)
7. Create your shortcut : For some reason Apple has blocked CMD+#'s even though those shortcuts aren't used in the app as far as I diagnosed. I was able to successfully replicate your desired results with ⌥+⌘+3, but your computer, do what makes you happy
8. Enjoy for $0

EDIT If you use ⌥+⌘ + 1/2 they are already reserved for the History Menu (so you should change these in Keyboard > Shortcuts by altering the keyboard board shortcut for that menu title) and then ⌥+⌘ + 1/2 will be up for grabs again

RE-EDIT Apparently ⌘+#s are reserved for jumping to bookmarks. Since I don't have any it didn't prompt anything on my end. But again, all menu title keyboard short cuts can be altered with no work at all in the keyboard shortcut area in system preferences. So looks like you can use your Command + 1-9 buttons