Fix command-line override of the User-Agent product component (see issue #2622).

When the NetworkService is enabled the U-A string is configured via
SystemNetworkContextManager::CreateDefaultNetworkContextParams, which calls
chrome_content_browser_client.cc GetUserAgent(). This change modifies the Chrome
implementation to match CEF, so that the U-A product component can still be
overridden via the `--product-version` command-line flag.

To test: Verify that chrome://version, navigator.userAgent (JS executed from
DevTools console) and network requests (headers shown in DevTools Network tab)
show the expected User-Agent value in the following cases:
- Running `cefclient --enable-network-service --user-agent="<value>"`
- Running `cefclient --enable-network-service --product-version="<value>"`

patch/patch.cfg

@@ -244,6 +244,12 @@ patches = [
     # https://bitbucket.org/chromiumembedded/cef/issues/2613
     'name': 'chrome_browser_net_export',
   },
+  {
+    # Support override of the User-Agent product component when NetworkService
+    # is enabled.
+    # https://bitbucket.org/chromiumembedded/cef/issues/2622
+    'name': 'chrome_browser_product_override',
+  },
   {
     # Allow CEF to share Chrome plugin loading code.
     'name': 'chrome_plugins',