Move initializer outside of performance callback

[lhoffbeck]

PR Summary:

Fix JS error when updating cart quantity

Why are these changes introduced?

Updating line item quantity in cart was causing a JS error. Source of the problem was ultimately that we were referencing a variable outside of it's defined scope.

Demo: https://screenshot.click/28-50-m3p8k-e1xpf.mp4

What approach did you take?

Confirmed that perf measurement doesn't need to include JSON parsing, moved the initializer to the parent scope

Demo links

• https://admin.shopify.com/store/os2-demo/themes/174907785238/editor?category=gid%3A%2F%2Fshopify%2FOnlineStoreThemeSettingsCategory%2FCart%3Ftheme_id%3D174907785238%26first_setting_id%3Dcart_type&previewPath=%2Fcart
• Check that you can change a line item quantity in cart. Line item and total are updated correctly and no error is displayed

Checklist

• Added PR summary for release notes
• Requested review from UX (Only for changes that are affecting the experience or perceivable visual details)
• Created a ticket for the help.shopify.com documentation team about updates to theme settings. (Internal-only task)
• Followed theme code principles
• Linted with Theme Check
• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility