fix(Chart): Wait Tick Before Initializing Chart

What does this MR do?

Note that mounted does not guarantee that all child components have also been mounted. If you want to wait until the entire view has been rendered, you can use vm.$nextTick inside of mounted...

-- https://v3.vuejs.org/api/options-lifecycle-hooks.html#mounted

In some instances, when trying to use charts within slots of other components, the div that the chart initializes to is not present, so creating the chart fails. However, waiting a tick guarentees that the div is present, so the chart succeeds.

closes #1131 (closed)

before 😢	after 😁

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team