This refactoring makes it easier to write advanced tests and
gives full control over what's happening to the test code.
It also forces description for every test.

BUG=none

Review-Url: https://codereview.chromium.org/2891213002
Cr-Commit-Position: refs/heads/master@{#45412}

- added InspectorTest.setupInjectedScriptEnvironment method which mutates current context,
- clear prototype of InjectedScript function and domAttributesWithObservableSideEffectOnGet.

Second point increases chances that injected-script-source would be successfully compiled.

BUG=chromium:693338
R=dgozman@chromium.org,luoe@chromium.org

Review-Url: https://codereview.chromium.org/2770823003
Cr-Commit-Position: refs/heads/master@{#44081}