Modularize components/webapps/isolated_web_apps/ further

ash/ and chrome/browser/ash/ depend on the isolated_web_apps component
just for accessing the IwaKeyDistributionInfoProvider; this CL factors
the provider into its own source_set to limit the dependency on other
IWA files. This is necessary to allow the main source_set to depend on
content/public/browser later on for things like BrowserContext*,
WebContents* or FrameTreeNodeId (these cannot be pulled in by ash/).

Bug: 431980377
Change-Id: I80c91eff1a3ac18ca7bcebaa069fe6c4bdb15bf6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6760873
Commit-Queue: Andrew Rayskiy <greengrape@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Auto-Submit: Andrew Rayskiy <greengrape@google.com>
Cr-Commit-Position: refs/heads/main@{#1489594}