Normalize all tab id values to integer

[gorhill]

In light of of regression #3425.

Tab id values are natively integer values, but they are currently normalized to strings internally on the platform-independent code.

The reason for this is historical: tab id values were used as keys in Object used as dictionary (Map/Set were not available back then). When an integer is used as a key, it is converted to string internally by javascript, and this means when iterating through the items in the Object-based dictionary, the tab id key value was a string. Instead of converting the string back to integer, back then I chose to normalize all tab id values to string.

This issue is to normalize tab id values to their native integer representation, and convert Object-based dictionaries to Maps where needed so as to avoid the implicit javascript conversion of tab id values to string.

The behind-the-scene scope will be represented with the integer -1.

The no tab found value will be 0.

[jspenguin2017]

Are you absolutely sure Chrome will never assign a tab to have ID 0?
When there is no tab, Chrome will set the tab ID to be the constant chrome.tabs.TAB_ID_NONE which happens to be -1. It is safe enough to assume that all tab IDs are larger than -1 although there is no documentation on that. I think -2 (or null, Infinity, etc) are safer than 0.