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We need to tell the location of dwo files to gdb too, to load debug information
correctly.
Reported in
https://groups.google.com/a/chromium.org/d/msg/chromium-dev/Y_2aYS6_DXA/qQCeeznpCwAJ

I also changed to use event handler for newly loaded objfiles.

Bug: crbug.com/439949, crbug.com/603286
Change-Id: Ib0b52c5553578333cd0dd44f11a9117345622844
Reviewed-on: https://chromium-review.googlesource.com/1068785Reviewed-by: Aleks Totic <atotic@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561632}