- 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}