Add a method to ExtractDelegate for querying if the extraction has been cancelled.

dependencies/pdfium/macx/VERSION

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+YACReader for macOS uses a precompiled single static library version of pdfium.
+
+pdfium branch used for building: chromium/3071 (current beta)
+
+build parameters used (gn args):
+
+pdf_enable_xfa = false
+pdf_enable_v8 = false
+pdf_is_complete_lib = true
+is_official_build = true
+symbol_level = 0
+ 
+Instructions on building pdfium can be found at https://pdfium.googlesource.com/pdfium
+
+It is recommended to always use the branch the current stable version of chromium uses.
+To get the pdfium branch corresponding to Chromium stable, look at 
+http://omahaproxy.appspot.com and search for the true_branch variable associated with
+the current stable dev channel of Chromium.