[WL] Add renderer-side phishing classifier

Implement the code needed for the renderer-side phishing classifier.

In particular:
- extend WebLayerRenderFrameObserver::CapturePageText to take a
  TextCaptureType,
- call CapturePageText when frame finishes loading (in addition to
  parsing),
- add/initialize PhishingClassifierDelegate as member of
  WebLayerRenderFrameObserver,
- ensure to capture pages (from CapturePageText) by phishing
  classifier,
- amend to capture pages through translate agent only when in
  preliminary capture mode,
- add kWebLayerClientSidePhishingDetection feature.

BUG=1149411,1115700

Change-Id: If98539211044e8c647eda13fbd65bfafdfcf054a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2544608
Commit-Queue: Tim Volodine <timvolodine@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830053}