Add Blink support for showing image placeholders using range requests.

third_party/WebKit/Source/core/loader/ProgressTracker.cpp

Index: third_party/WebKit/Source/core/loader/ProgressTracker.cpp
diff --git a/third_party/WebKit/Source/core/loader/ProgressTracker.cpp b/third_party/WebKit/Source/core/loader/ProgressTracker.cpp
index 69833e777b26c7a801b154337a6c9eda3a3a64ee..23721d408a8655bf7ce55a2d5487dddb75705c1f 100644
--- a/third_party/WebKit/Source/core/loader/ProgressTracker.cpp
+++ b/third_party/WebKit/Source/core/loader/ProgressTracker.cpp
@@ -141,7 +141,6 @@ void ProgressTracker::willStartLoading(unsigned long identifier,
           ProgressBarCompletion::LoadEvent &&
       (m_finishedParsing || priority < ResourceLoadPriorityHigh))
     return;
-  DCHECK(!m_progressItems.get(identifier));

[Nate Chapin, 2016/10/21 18:23:55]

I missed this...when does this DCHECK get hit? This implies that we're reloading
the ImageResource without fulling cancelling it first.

[sclittle, 2016/10/21 18:57:25]

The ProgressTracker doesn't remove resources from its |m_progressItems| map as
the resources complete or are cancelled, so this DCHECK would fire for any image
that is being tracked by the ProgressTracker and is reloaded for some reason
while the page is still loading, e.g. if the image subrange didn't contain the
dimensions.

I'm pretty sure this DCHECK could also trigger even before this CL if the user's
in Lo-Fi mode and causes the image to be reloaded while the page is still
loading, although I haven't actually reproduced that myself.

[Nate Chapin, 2016/10/21 19:13:10]

Fair enough. I'm debating whether it's better to drop the DCHECK or to instead
check that if there is a ProgressItem, it's "finished" (it's estimated and
received byte counts are the same).

   m_progressItems.set(
       identifier,
       wrapUnique(new ProgressItem(progressItemDefaultEstimatedLength)));