add nativeWebScreenshot

[KazuCocoa]

Just today, I noticed Espresso driver had no nativeWebScreenshot. We cannot get screenshot in webview context. uiautomator2 also cannot get screenshot in webview if nativeWebScreenshot is not enabled.

(I will add test code later.)

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• Error when I tried to get screenshot in webview context

[HTTP] --> GET /wd/hub/session/1a3fc9a9-0684-40b4-9add-32a416e728e6/screenshot
[HTTP] {}
[W3C (1a3fc9a9)] Driver proxy active, passing request on via HTTP proxy
[debug] [WD Proxy] Matched '/wd/hub/session/1a3fc9a9-0684-40b4-9add-32a416e728e6/screenshot' to command name 'getScreenshot'
[debug] [WD Proxy] Proxying [GET /wd/hub/session/1a3fc9a9-0684-40b4-9add-32a416e728e6/screenshot] to [GET http://127.0.0.1:8000/wd/hub/session/1564e8c3fe4d676d33410f6d2c5d0e3f/screenshot] with body: {}
[WD Proxy] Got an unexpected response: {"code":"ESOCKETTIMEDOUT","connect":false}
[debug] [W3C (1a3fc9a9)] Encountered internal error running command: Error: Could not proxy. Proxy error: Could not proxy command to remote server. Original error: Error: ESOCKETTIMEDOUT
[debug] [W3C (1a3fc9a9)]     at doJwpProxy (/Users/kazuaki/GitHub/appium/node_modules/appium-base-driver/lib/protocol/protocol.js:547:13)
[HTTP] <-- GET /wd/hub/session/1a3fc9a9-0684-40b4-9add-32a416e728e6/screenshot 500 240023 ms - 705
[HTTP]

Then, I could not find error message in espresso server...

[KazuCocoa]

feel free to merge this to include this in 1.11, since I have no permission. (Or could you give me a permission to merge this? :) )