Forward kWebAppEnableLinkCapturing flag into Blink as WebAppLinkCapturing flag

This CL simplifies the declarative link capturing flag situation by
making WebAppLinkCapturing inherit the value of
kWebAppEnableLinkCapturing. This means Blink only needs to read one flag
to determine whether this feature is enabled.

This is required for using implied_by: ['WebAppLinkCapturing'] in
runtime_enabled_features.json5 in a future CL.


Bug: 1163398
Change-Id: Ia0593cf9cf572b865a9c86497fb9e49fe033e58a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2966419
Reviewed-by: Nate Chapin <japhet@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#893685}