Revert of Do not initialize v8::Context on |ScriptController::updateDocument|. (https://codereview.chromium.org/263583002/)

Reason for revert:
broke interactive_ui_tests:

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Original issue's description:
> Do not initialize |v8::Context| on |ScriptController::updateDocument|.
> 
> 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

TBR=haraken@chromium.org,jochen@chromium.org,dcarney@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=368548, 368555

Review URL: https://codereview.chromium.org/268623003

git-svn-id: svn://svn.chromium.org/blink/trunk@173060 bbb929c8-8fbe-4397-9dbb-9b2b20218538