Search - change browser supplied 'x' icon

[jcfilben]

In webkit browsers when an input has type='search' a blue 'x' is automatically added to the input.

During office hours 9/14 this issue was brought up. There was a desire to override the default blue 'x' and replace this with a clear button that is more aligned with our theming. This would ensure users get the same experience across browsers.

Issue brought up by Bhim Dutta

Related to https://github.com/grommet/grommet/issues/5257