Cherry-pick "[android_webview] Hardcode the default text encoding to UTF-8."

The WebView will use a fixed default value for the text encoding instead
of getting it from the platform. This is in attempt to unify behavior
between devices.

BUG=422366

Review URL: https://codereview.chromium.org/639113004

Cr-Commit-Position: refs/heads/master@{#299287}

Conflicts:
	DEPS
	android_webview/buildbot/aosp_manifest.xml
	android_webview/renderer/aw_content_renderer_client.h
	chrome/renderer/chrome_content_renderer_client.cc
	chrome/renderer/chrome_content_renderer_client.h

Change-Id: I11c7c351b8640d3657f31b9154574dbae0979104
14 files changed