Don't show video fullscreen until DidEnterFullscreen() is called, and disable fullscreen for non-media elements for android webview

This change is copied from https://codereview.chromium.org/284913002/
However, the previous change breaks android webview because android webview doesn't support fullscreen except media.
AwWebContentsDelegate::ToggleFullscreenModeForTab is actually not implemented in webview.
To solve the webview issue, this change adds a flag to disallow elements other than video to enter fullscreen.
When this flag is enabled, webview will only respond to fullscreen calls from the video element and can thus send the resize IPC to complete the will/didEnterfullscreen call.

BUG=367346

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=177458

Review URL: https://codereview.chromium.org/290643005

git-svn-id: svn://svn.chromium.org/blink/trunk@177626 bbb929c8-8fbe-4397-9dbb-9b2b20218538
IDBTransactionMode is now an enum

Converting since Blink's IDL code generator supports enumerations.

BUG=358067

Review URL: https://codereview.chromium.org/376703003

git-svn-id: svn://svn.chromium.org/blink/trunk@177627 bbb929c8-8fbe-4397-9dbb-9b2b20218538