DevTools: Hide JS frames on timeline behind the experiment.

Review URL: https://codereview.chromium.org/424383003

git-svn-id: svn://svn.chromium.org/blink/trunk@179224 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Drop the 'WebURLRequest::TargetType' enum.

We no longer use this enum in Chromium; everything has been ported over
to using '::RequestContext' and '::FrameType', so we can safely remove
both the enum and the translation machinery in '::setTargetType'.

BUG=390497
TBR=kbr@chromium.org

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This is a reland of https://codereview.chromium.org/417483002, which was
reverted in https://codereview.chromium.org/422343003 due to suspicious
flakiness. As it turns out, this series of patches wasn't the cause.
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Review URL: https://codereview.chromium.org/425233002

git-svn-id: svn://svn.chromium.org/blink/trunk@179225 bbb929c8-8fbe-4397-9dbb-9b2b20218538