v4: fullscreen background content is not behind transparent header

[brandyscarney]

Bug Report

Ionic Info

master branch

Describe the Bug
Using the following code:

<ion-header>
  <ion-toolbar style="--background: transparent">
    <ion-title>Toolbar</ion-title>
  </ion-toolbar>
</ion-header>

<ion-content fullscreen style="--color: white; --background: url('https://images.unsplash.com/photo-1530579807716-d82c0991629a?ixlib=rb-0.3.5&ixid=eyJhcHBfaWQiOjEyMDd9&s=8c708171e9cdb8ae7ed04f8e37e2808c&auto=format&fit=crop&w=934&q=80') no-repeat center center fixed">
  Content
</ion-content>

With the current way master is, the toolbar does not look transparent because it still appears to have a background, see the below image.

If we move the background declaration to the inner scroll element, we get the following:

cc @manucorporat

[ionitron-bot]

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