Crash caused by RenderThreadImpl::OnMemoryPressure being called before
blink is initialized. Fix by skipping the crashing code if blink has not
been initialized.
BUG=398768
Review URL: https://codereview.chromium.org/437623003
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287167 0039d316-1c4b-4281-b951-d872f2087c98