Implement <webview>.addContentScript/removeContentScript API [3]

This patch enables the code injection for
<webview>.addContentScript/removeContentScript API.

This is the third 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)

BUG=461052

Review URL: https://codereview.chromium.org/1058113002

Cr-Commit-Position: refs/heads/master@{#327037}
6 files changed