Apply Chromium style fixes.

This addresses several minor code quality issues that are validated
in Chromium, but not yet applied to ANGLE:

* constructors and destructors must be defined out-of-line
* auto is not allowed for simple pointer types
* use override everywhere instead of virtual
* virtual functions must also be defined out-of-line

Slightly reduces binary size for me (~2k on Win, 150k on Linux).

Bug: angleproject:1569
Change-Id: I073ca3365188caf5f29fb28d9eb207903c1843e6
Reviewed-on: https://chromium-review.googlesource.com/779959
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>

util/OSWindow.cpp

@@ -271,6 +271,11 @@ int OSWindow::getHeight() const
     return mHeight;
 }
 
+bool OSWindow::takeScreenshot(uint8_t *pixelData)
+{
+    return false;
+}
+
 bool OSWindow::popEvent(Event *event)
 {
     if (mEvents.size() > 0 && event)