Fix mismatch about tabId in code and webrequest API documentation if requests don't belong to a tab

Fix mismatch about tabId in code and webrequest API documentation if requests don't belong to a tab

This CL updates the webrequest API documentation to state that requests that cannot be associated with a tab are labeled with "tabId = -1" instead of "tabId = null".

I chose to update the documentation rather than the code for two reasons:
- As 0 is a valid value for the tabId, we would need to distinguish between 0 and null.
- For the RequestFilter type, 'tabId' is an optional parameter. I.e. we would need to distinguish between 0, null and undefined.

BUG=93373
TEST=no


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=98289

[battre, 2011-08-25 09:32:21]

Please review. Thanks.

[Matt Perry, 2011-08-25 18:08:17]

LGTM

[commit-bot: I haz the power, 2011-08-25 20:32:16]

Change committed as 98289