Bug 1540573 - P6. Use frugal preloading of media data when on cellular, otherwise aggressive. r=jya

We're allowed to take some liberties as to what the default value and behaviour
we assume for the 'preload' attribute on HTMLMediaElement by the spec. On
desktop we assumed preload="metadata", while on mobile we assumed the default
of preload="none" to save data. On mobile we also assumed that preload="auto"
meant preload="metadata".

I think it makes sense to instead of always assuming that data on Android is
always expensive, we can instead detect if we're running on a cellular connection,
and preload frugally then, otherwise aggressively.

Differential Revision: https://phabricator.services.mozilla.com/D26235

--HG--
extra : moz-landing-system : lando

dom/html/HTMLMediaElement.cpp

@@ -2455,6 +2455,27 @@ bool HTMLMediaElement::AllowedToPlay() const {
   return AutoplayPolicy::IsAllowedToPlay(*this);
 }
 
+uint32_t HTMLMediaElement::GetPreloadDefault() const {
+  if (mMediaSource) {
+    return HTMLMediaElement::PRELOAD_ATTR_METADATA;
+  }
+  if (OnCellularConnection()) {
+    return Preferences::GetInt("media.preload.default.cellular",
+                               HTMLMediaElement::PRELOAD_ATTR_NONE);
+  }
+  return Preferences::GetInt("media.preload.default",
+                             HTMLMediaElement::PRELOAD_ATTR_METADATA);
+}
+
+uint32_t HTMLMediaElement::GetPreloadDefaultAuto() const {
+  if (OnCellularConnection()) {
+    return Preferences::GetInt("media.preload.auto.cellular",
+                               HTMLMediaElement::PRELOAD_ATTR_METADATA);
+  }
+  return Preferences::GetInt("media.preload.auto",
+                             HTMLMediaElement::PRELOAD_ENOUGH);
+}
+
 void HTMLMediaElement::UpdatePreloadAction() {
   PreloadAction nextAction = PRELOAD_UNDEFINED;
   // If autoplay is set, or we're playing, we should always preload data,
@@ -2469,13 +2490,8 @@ void HTMLMediaElement::UpdatePreloadAction() {
         mAttrs.GetAttr(nsGkAtoms::preload, kNameSpaceID_None);
     // MSE doesn't work if preload is none, so it ignores the pref when src is
     // from MSE.
-    uint32_t preloadDefault =
-        mMediaSource
-            ? HTMLMediaElement::PRELOAD_ATTR_METADATA
-            : Preferences::GetInt("media.preload.default",
-                                  HTMLMediaElement::PRELOAD_ATTR_METADATA);
-    uint32_t preloadAuto = Preferences::GetInt(
-        "media.preload.auto", HTMLMediaElement::PRELOAD_ENOUGH);
+    uint32_t preloadDefault = GetPreloadDefault();
+    uint32_t preloadAuto = GetPreloadDefaultAuto();
     if (!val) {
       // Attribute is not set. Use the preload action specified by the
       // media.preload.default pref, or just preload metadata if not present.

dom/html/HTMLMediaElement.h

@@ -1737,6 +1737,9 @@ class HTMLMediaElement : public nsGenericHTMLElement,
 
   virtual void MaybeBeginCloningVisually(){};
 
+  uint32_t GetPreloadDefault() const;
+  uint32_t GetPreloadDefaultAuto() const;
+
   /**
    * This function is called by AfterSetAttr and OnAttrSetButNotChanged.
    * It will not be called if the value is being unset.

mobile/android/app/mobile.js

@@ -550,9 +550,6 @@ pref("browser.chrome.toolbar_tips", false);
 // don't allow meta-refresh when backgrounded
 pref("browser.meta_refresh_when_inactive.disabled", true);
 
-// prevent video elements from preloading too much data
-pref("media.preload.default", 1); // default to preload none
-pref("media.preload.auto", 2);    // preload metadata if preload=auto
 // On mobile we throttle the download once the readahead_limit is hit
 // if we're using a cellular connection, even if the download is slow.
 // This is to preserve battery and data.