Specify output name for pdfium build component

Starting with crrev.com/1096860, the Chromium build rules can modify the
resulting name of the built component.  In places where loading the
pdfium component library is hardcoded, the options are:
- modify the name in the source code where it is loaded, to follow
  the new naming scheme;
- use output_name in PDFium build rules to explicitly specify the
  name of the library, so that Chromium won't modify its name.

Use of the output_name approach is easier to understand when seeing the
pdfium component name hard-coded in source.  It also protects such code
blocks from any further modifications to the build rule.

Bug: chromium:1410951
Change-Id: Ib9a2375c760d58a521815dc08f3b8c5d31d3f36a
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/103501
Commit-Queue: Alan Screen <awscreen@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 0cd3fa9..62210c2 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -342,6 +342,7 @@
 }
 
 component("pdfium") {
+  output_name = "pdfium"
   libs = []
   configs += [ ":pdfium_strict_config" ]
   public_configs = [ ":pdfium_public_config" ]