How to reload a chrome extension automatically?

The extension can reload itself, by calling chrome.runtime.reload(), so it's a matter of triggering the extension to do it.

One method that worked for me, is to watch for tabs' onUpdated event and look for a specific URL (you have come up with), e.g. http://localhost/reloadX?id=....

This is the sample code (to be placed in background.js):

var myReloadURL = 'http://localhost/reloadX?id='
                  + chrome.i18n.getMessage('@@extension_id');

chrome.tabs.onUpdated.addListener(function(tabId, info, tab) {
    if (info.url === myReloadURL) {
        chrome.tabs.remove(tabId);
        chrome.runtime.reload();
    }
});

Additional permissions (to be declared in manifest.json):

...
"permissions": [
    ...
    "tabs",
    "http://localhost/reloadX?id=*"

_{myReloadURL is arbitrary and can be any URL, just doesn't have to be a real URL or the resource will be rendered unreachable.}

Now, in order to reload your extension, you need to open the following address in Chrome:
http://localhost/reloadX?id=<your_extension_id>

It is up to you to choose how to trigger that on save. It could be an on-save hook in your editor, a custom grunt-watch task (since you seem to be familiar with grunt) etc .

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(BTW, you don't need to reload the chrome://extensions page. It suffices to reload the extension.)