Add a Setting to make iframes load at VeryLow priority, disabled by default.

Source/web/WebSettingsImpl.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 170 matching lines @@
     virtual void setViewportMetaEnabled(bool) override;
     virtual void setViewportMetaLayoutSizeQuirk(bool) override;
     virtual void setViewportMetaMergeContentQuirk(bool) override;
     virtual void setViewportMetaNonUserScalableQuirk(bool) override;
     virtual void setViewportMetaZeroValuesQuirk(bool) override;
     virtual void setWebAudioEnabled(bool) override;
     virtual void setWebGLErrorsToConsoleEnabled(bool) override;
     virtual void setWebSecurityEnabled(bool) override;
     virtual void setWideViewportQuirkEnabled(bool) override;
     virtual void setXSSAuditorEnabled(bool) override;
+    void setLowPriorityIframes(bool) override;

[Bryan McQuade, 2015/06/19 20:35:34]

alternatively to this, we can use --blink-settings flag, which goes through
WebSettings::setFromStrings to set the value in the Settings instance. That
means you don't need to expose setters on WbSettings/WebSettingsImpl. For
running a field trial, I've found that it's easy to use the blink-settings flag
(see
https://chromium.googlesource.com/chromium/src/+/5d2d9cf3248dbefa5eab41dfc7c2...
for an example of another field trial that uses this flag), so unless you need
this for other purposes, you could consider removing it from this patch.

 
     bool showFPSCounter() const { return m_showFPSCounter; }
     bool showPaintRects() const { return m_showPaintRects; }
     bool renderVSyncNotificationEnabled() const { return m_renderVSyncNotificationEnabled; }
     bool autoZoomFocusedNodeToLegibleScale() const { return m_autoZoomFocusedNodeToLegibleScale; }
     bool doubleTapToZoomEnabled() const;
     bool perTilePaintingEnabled() const { return m_perTilePaintingEnabled; }
     bool supportDeprecatedTargetDensityDPI() const { return m_supportDeprecatedTargetDensityDPI; }
     bool viewportMetaEnabled() const;
     bool viewportMetaLayoutSizeQuirk() const { return m_viewportMetaLayoutSizeQuirk; }
@@ skipping 24 matching lines @@
     // This quirk is to maintain compatibility with Android apps built on
     // the Android SDK prior to and including version 18. Presumably, this
     // can be removed any time after 2015. See http://crbug.com/313754.
     bool m_clobberUserAgentInitialScaleQuirk;
     bool m_mainFrameResizesAreOrientationChanges;
 };
 
 } // namespace blink
 
 #endif