Regression in pdf printing font subsetting in Qt Webengine 5.15.2

Fonts used while printing to pdf are no longer being subset leading to much larger output file sizes. This is a regression from 5.15.1 to 5.15.2. Below is a trivial 5 line test script using PyQt to replicate. When run against Qt 5.15.1 it produces a 12KB PDF, when run against Qt 5.15.2 it produces a 207KB pdf. Using any tool to examine the PDF shows the difference is caused by a much larger embedded font. You can use, for example, mutool extract to verify this. The attached example simply creates a PDF with three letters in it, abc, rendered using the free Liberation Serif font.

```

from PyQt5.QtWebEngineWidgets import QWebEnginePage
from PyQt5.Qt import QApplication
app = QApplication([])
p = QWebEnginePage()
p.pdfPrintingFinished.connect(app.quit)

p.loadFinished.connect(lambda ok: p.printToPdf('test.pdf'))
p.setHtml('''<p style="font-family: 'Liberation Serif'">abc''')
app.exec_()

```

Am sure this can be trivially converted to C++. Have tested it on Linux and macOS but probably exists on other platforms too.