Track the region where text is painted.

Source/core/rendering/RenderImage.cpp

Index: Source/core/rendering/RenderImage.cpp
diff --git a/Source/core/rendering/RenderImage.cpp b/Source/core/rendering/RenderImage.cpp
index 60ead09bd296a5ef2f7db0ec1266ab95fc38963c..6d86bd57a0727bad28c77ff19b772da2dc634577 100644
--- a/Source/core/rendering/RenderImage.cpp
+++ b/Source/core/rendering/RenderImage.cpp
@@ -341,18 +341,20 @@ void RenderImage::paintReplaced(PaintInfo& paintInfo, const LayoutPoint& paintOf
                 const Font& font = style()->font();
                 const FontMetrics& fontMetrics = font.fontMetrics();
                 LayoutUnit ascent = fontMetrics.ascent();
-                LayoutPoint altTextOffset = paintOffset;
-                altTextOffset.move(leftBorder + leftPad + (paddingWidth / 2) - borderWidth, topBorder + topPad + ascent + (paddingHeight / 2) - borderWidth);
+                LayoutPoint textRectOrigin = paintOffset;
+                textRectOrigin.move(leftBorder + leftPad + (paddingWidth / 2) - borderWidth, topBorder + topPad + (paddingHeight / 2) - borderWidth);
+                LayoutPoint textOrigin(textRectOrigin.x(), textRectOrigin.y() + ascent);
 
                 // Only draw the alt text if it'll fit within the content box,
                 // and only if it fits above the error image.
                 TextRun textRun = RenderBlock::constructTextRun(this, font, text, style());
                 LayoutUnit textWidth = font.width(textRun);
+                FloatRect textRect(textRectOrigin, FloatSize(textWidth, fontMetrics.height()));
                 if (errorPictureDrawn) {
                     if (usableWidth >= textWidth && fontMetrics.height() <= imageOffset.height())
-                        context->drawText(font, textRun, altTextOffset);
+                        context->drawText(font, textRun, textOrigin, textRect);
                 } else if (usableWidth >= textWidth && usableHeight >= fontMetrics.height())
-                    context->drawText(font, textRun, altTextOffset);
+                    context->drawText(font, textRun, textOrigin, textRect);
             }
         }
     } else if (m_imageResource->hasImage() && cWidth > 0 && cHeight > 0) {