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

Source/web/WebSettingsImpl.h

Index: Source/web/WebSettingsImpl.h
diff --git a/Source/web/WebSettingsImpl.h b/Source/web/WebSettingsImpl.h
index bab2ed72d4e79efc304306b1d76946ed837f043f..6e979e52a090897db6c1f3dc3df5353aef80233d 100644
--- a/Source/web/WebSettingsImpl.h
+++ b/Source/web/WebSettingsImpl.h
@@ -188,6 +188,7 @@ public:
     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; }