Implement <webview>.addContentScript/removeContentScript API [1].

This patch includes the changes that enables <webview>.addContentScript/removeContentScript API  work on extensions.

This is the first patch in a series of patches:
1) Implement <webview>.addContentScript/removeContentScript API [1] (https://codereview.chromium.org/959413003)
2) Implement <webview>.addContentScript/removeContentScript API [2] (https://codereview.chromium.org/1056533002)
3) Implement <webview>.addContentScript/removeContentScript API [3] (https://codereview.chromium.org/1058113002)

TBR=asvitkine@chromium.org
BUG=461052

Review URL: https://codereview.chromium.org/959413003

Cr-Commit-Position: refs/heads/master@{#325492}
28 files changed