Cache all chrome.loadTimes info before passing them to setters.

The setters can invalidate the pointers frame, data_source and document_state.

BUG=549251

Review URL: https://codereview.chromium.org/1422753007

Cr-Commit-Position: refs/heads/master@{#357201}
  LoadTimesExtensionWrapper() :
    v8::Extension(kLoadTimesExtensionName,
      "var chrome;"
      "if (!chrome)"
      "  chrome = {};"

  LoadTimesExtensionWrapper() :
    v8::Extension(kLoadTimesExtensionName,
      "var chrome;"
      "if (!chrome)"
      "  chrome = {};"
