HTTPRequest get_body_size() returns -1 in HTML5

### Tested versions

v3.5.3.stable.official [6c814135b]

### System information

MacOS Sonoma 14.1.2.
Browser tested: Chrome, Firefox.

### Issue description

When calling the `HTTPRequest.get_body_size()` function, it returns -1 only in HTML5, while it works as expected in MacOS.

### Steps to reproduce

Call the function in `get_body_size()` while in HTML5.

### Minimal reproduction project (MRP)

N/A

Start by reproducing HTTPRequest.get_body_size() in an HTML5 build using Chrome or Firefox, then compare its result with the macOS behavior described in the issue. Trace the HTML5-specific HTTPRequest path and verify that the function returns the correct body size instead of -1; no minimal reproduction project or specific source file is provided.

Written by the indexing model from the issue text.