LinkPreview: Add alt+hover and long press triggers

This CL adds alt+hover and long press triggers, and a feature param
"trigger_type" for LinkPreview to control available trigger type.

Before this CL, Link Preview used only Alt+click trigger. Now it is
controlled by features::LinkPreviewTriggerType::kAltClick.

For this purpose, this CL introduces

- WebLinkPreviewTriggerer: receives signals and determine to trigger
  Link Preview.
- WebLinkPreviewInitiator: mediates initiation request to blink side.

Fixed: b:314233999, 1522049
Change-Id: Ic3f2bc4108ce514156086c473a9727d7a10c27d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5107229


Reviewed-by: Alex Ilin <alexilin@chromium.org>
Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Ken Okada <kenoss@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1266909}