Add FrameHost mojo service

Adds new frame-level service with one initial method to handle host zoom
level.

This moves zoom level supply from async_resource_handler.cc to being a
request made when render_frame_impl handles a willSendRequest.

The goal of this change is to remove the dependency of
content/browser/loader on the rest of content/browser in particular
here, removing the use of c/b/host_zoom_map_impl.h in
content/browser/loader/async_resource_handler.cc.

BUG=598073,609607
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/1964273002
Cr-Original-Commit-Position: refs/heads/master@{#394547}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 68c6f2ce16d9807b5cb82679099c82c40f39e911

android/pylib/gtest/filter/content_browsertests_disabled

@@ -35,7 +35,7 @@ RenderViewImplTest.InsertCharacters
 RenderViewImplTest.OnHandleKeyboardEvent
 RenderViewImplTest.OnNavStateChanged
 # ZoomLevel is not used on Android
-RenderViewImplTest.ZoomLimit
+RenderFrameImplTest.ZoomLimit
 RendererAccessibilityTest.SendFullAccessibilityTreeOnReload
 RendererAccessibilityTest.HideAccessibilityObject
 RendererAccessibilityTest.ShowAccessibilityObject