Reland: Do not initialize |v8::Context| on |ScriptController::updateDocument|.

> The reason for reverting is: broke interactive_ui_tests:
> 
> Referrer
> SearchReusesInstantTab
> TypedSearchURLDoesntReuseInstantTab.
I fixed these tests in https://src.chromium.org/viewvc/chrome?view=rev&revision=273996 .

Original Description:

Initializing |v8::Context| is a heavy operation, and should be avoided if necessary. This is meant to be delayed until first time |toV8Context| is called, but |ScriptController::updateDocument| was forcing creation.

This patch fixes the if branch so that it would not create a |v8::Context| if it does not exist.

BUG=368548, 368555

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=173044

Review URL: https://codereview.chromium.org/263583002

git-svn-id: svn://svn.chromium.org/blink/trunk@175240 bbb929c8-8fbe-4397-9dbb-9b2b20218538